Direct Blowback in WWII submachine guns


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Booner63
February 18, 2014, 07:53 PM
So I've been taking a look at some World War II era submachine guns (such as the M3, Thompson M1, MP-40, and Sten), and I noticed that unlike modern submachine guns (such as the H&K MP5), these have no delayed blowback method- they are simply direct blowback. I thought I might be missing something, seeing as the general round cap for direct blowback is 9mm Makarov, and anything larger than that used some form of delay (i.e. short action, roller delay, lever delay, gas operation, etc.). So, I did some looking into this and found that these firearms were indeed direct blowback. How is this possible with firearms firing rounds of this size?
My logic was that since inertia is the only thing holding the round in place long enough for the pressure to drop, a larger round would require a more massive bolt. Since these firearms are open bolt, the recoil spring needs to be stiff enough to push the bolt fast enough to ignite the primers, as well as simply force the bullet into battery.
So using that logic, there are three conclusions I determined could be drawn:

1. World War II era submachine guns lost more gas than normal out of the ejection port

2. World War II era submachine guns were extremely difficult to cock

or

3. Whoever determined that 9x18 Makarov is the largest advisable to use in direct blowback designs is mistaken.

Am I on the right track? If so, which if these conclusions is correct? If not, can you explain this to me?

Thanks



Oh, and I assume that this is the correct categorization for this thread, since it specifically deals with open bolt firearms, which are now banned without permit (although this principle applies to all blowback firearms).

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Sam1911
February 18, 2014, 08:11 PM
There is an effect known as "Advanced Primer Ignition" wherein the primer is ignited just before the bolt fully closes, thus some of the energy of the charge is expended in overcoming the forward momentum of the bolt, before then driving it backward.

There are some large guns built with this clearly as a design feature. Some folks say that it occurs as part of an open bolt sub-gun's firing cycle as well, which is why they can use a slightly lighter bolt than would perhaps be expected.

I'm not 100% convinced that this is true (does the fixed firing pin stub actually dent the primer before the case stops abruptly against the headspace ledge in the chamber?), but it is a possibility.

rcmodel
February 18, 2014, 08:14 PM
There is a reason the .45 ACP Thompson weighed 11 pounds.

And the 9mm STEN weighed over 7.

The bolts weren't exactly light.

There was no more gas loss then normal either.
The bullet has to be gone & the pressure dropped before the case loses it grip on the chamber enough to be extracted.

Springs / cocking force were not excessively heavy either, because they play almost no part in holding the bolt closed longer.
And there was no hammer or striker spring in addition to the recoil spring that had to be cocked at the same time.

rc

Booner63
February 18, 2014, 08:21 PM
@Sam1911 I have heard of this, but I've never heard of it being used in submachine guns. Is the idea that it only happens as a coincidence of the design?

Booner63
February 18, 2014, 08:25 PM
@rcmodel
Even if the spring didn't help to keep the chamber sealed long enough, wouldn't it have to be stronger on a larger bolt in order to accelerate the bolt enough to load a cartridge into battery and (in the case of an open bolt) fire it?

Although I suppose I hadn't considered that closed bolts do require a spring for a firing mechanism.

Sam1911
February 18, 2014, 08:27 PM
Well, most resources don't mention it even in passing, and like I said, I don't know for sure that it DOES. So I suppose I don't believe it was an intended design feature, either.

If it happens, it most likely is a happy coincidence of the design, and simply one of the factors (like internal friction or magazine spring pressure, etc.) that were part of the mix as the bolt's weight was finalized to get the target cyclic rate in the last stages of development.

rcmodel
February 18, 2014, 08:30 PM
It was used in the German M38, MP40, MP44, and later on the Suomi M31, and later still on the UZI.

rc

Sam1911
February 18, 2014, 08:36 PM
So I've read. I'm still a little unsure of when in the motion of the case and bolt this happens, especially as it is after the bolt picks up the cartridge (otherwise the round would go off in the mag) and after that point the cartridge and bolt are moving at the same speed, until the cartridge stops in the chamber, just a millisecond before the bolt does.

When does the firing pin nub actually IMPACT the primer, and how, if that happens before the case has come to a stop in the chamber?

Booner63
February 18, 2014, 08:39 PM
I thought the MP44 was gas operated?

Sam1911
February 18, 2014, 08:41 PM
Yes.

rcmodel
February 18, 2014, 08:45 PM
Oops!

My typer got ahead of my thinker!

Rc

Booner63
February 18, 2014, 08:49 PM
I suppose I misread the "round cap" for direct blowback. The way I understood it, if a round was larger than 9x18 Makarov, then the spring large enough to push an appropriately large slide or bolt would be so strong, that the device would be virtually un-cockable. Could it be that 9x18 Mak is the general cap for handguns, the reason being that any larger and the handgun would be uncomfortably heavy?

rcmodel
February 18, 2014, 08:54 PM
Hi-Point firearms are blow-back.
.40 S&W
http://www.hi-pointfirearms.com/handguns/handguns_40sw.html

.45 ACP
http://www.hi-pointfirearms.com/handguns/handguns_45acp.html

.380 / 9mm MAK is about it for small pocket guns though.

rc

Booner63
February 18, 2014, 09:01 PM
Huh. So basically the 9mm Mak is considered the general cap for direct blowback not because it becomes difficult to manually cycle with any larger round, but because it becomes uncomfortably and inconveniently heavy for a handgun with a larger round. Hence why direct blowback can be used on shoulder fired weapons, where weight and one handed operation is not as much of a concern.

Sam1911
February 18, 2014, 09:06 PM
Certainly that is true. Hi Point firearms work, and work pretty well. Most shooters don't care for the heaviness and bulkiness required, though.

There are quite a few blowback-operated carbines, though, and they're considered perfectly acceptable because the weight and size are more in line with what one expects of a carbine.

Booner63
February 18, 2014, 09:10 PM
Thanks for the info. So I guess blowback delay systems like short action are a result steel being the "best" material to produce a firearm with at the time.

scramasax
February 18, 2014, 09:10 PM
Without going into details that I can't remember and have to look up there are several things that happen.

Yes you have a relatively stiff spring and heavy bolt balanced to absorb recoil energy. The case expands to fill and seal the chamber until the pressure drops to a level that is safe for the bolt to move. This is a mechanical locking action. By this time the projectile has already left the barrel. H&K used fluted chambers to increase the surface area for higher pressure rounds to use smaller bolts in some of their guns.

This is very simplified and I believe correct.

Also the Thompson and several other first generation submachine guns used other designs to make the bolt hesitate. Look up the Blish Block in the Thompson. These designs were found to be not needed. Yes advanced primer ignition was another of the ways that was used.

If you want to delve into this for a more complete picture do so. There are many good firearm books available that cover this. I've read a large number of them. I just don't remember the details.

Cheers,

ts

Booner63
February 18, 2014, 09:14 PM
From what I hear the blish lock was ineffective in practice, due to the limited adhesion of the bolt to the bolt face. Brilliant concept though.

stressed
February 18, 2014, 09:16 PM
@rcmodel
Even if the spring didn't help to keep the chamber sealed long enough, wouldn't it have to be stronger on a larger bolt in order to accelerate the bolt enough to load a cartridge into battery and (in the case of an open bolt) fire it?

Although I suppose I hadn't considered that closed bolts do require a spring for a firing mechanism.
One thing I notice on my blowback subguns/carbines is that the casings are fouled/dirty halfway down, as there is not a perfect seal, or bolt opening before pressure lowers.

Booner63
February 18, 2014, 09:18 PM
Is this found on all of your blowbacks, or only on ones that fire "larger" ammunition?

Sam1911
February 18, 2014, 09:20 PM
From what I hear the blish lock was ineffective in practice, due to the limited adhesion of the bolt to the bolt face. Brilliant concept though.

Yes, sort of a situation where whatever effects it was really producing in practice were insubstantial and unnecessary for the task at hand, but were just really amazingly insightful in theory.

H&K used fluted chambers to increase the surface area for higher pressure rounds to use smaller bolts in some of their guns.Actually, I think you've got that backward. The flutes let gas back around the brass to help "float" the brass off the chamber walls because the pressure is still pretty high as the bolt starts trying to extract.

rcmodel
February 18, 2014, 09:25 PM
You often get case fouling or blow-by on even locked-breech firearms.

There is a slight delay between firing, pressure rise, and complete case expansion to seal the chamber.

In the meantime, you may get some blow-by fouling on the outside of the cases on any type of action.

rc

Nom de Forum
February 18, 2014, 09:41 PM
[QUOTE]1. World War II era submachine guns lost more gas than normal out of the ejection port


They don't.

2. World War II era submachine guns were extremely difficult to cock


They aren't.


3. Whoever determined that 9x18 Makarov is the largest advisable to use in direct blowback designs is mistaken.

Could you please identify who determined this? Perhaps you misunderstood. You really can not compare blow back operation of SMGs to pistols because the enormous scale difference and type of use make the ergonomics of using blowback design very different. When you make small blowback operated SMGs (Mac 10, Mini-Uzi, etc.) you get all the bad features of blowback operated pistols and SMGs combined and very little benefit.

Jim K
February 18, 2014, 09:48 PM
1. There definitely is something called advanced primer ignition and it is part of the design of open bolt SMG's. When Numrich made semi-auto carbines using TSMG parts, they couldn't increase the bolt mass much because of the size limits of the receiver, so they had to use a much stronger bolt return spring. The open bolt FA weapons can be easily cocked with the little finger; the closed bolt semi's require a strong pull. The difference is API.

2. It is often believed that the Blish lock had no effect, but that is not true. It might not have had much, but it had some. When Savage tried to eliminate it as part of the design simplification that led to the M1 and M1A1 Thompsons, the receivers cracked at the rear due to the greater velocity and impact of the bolt, which the Blish lock had slowed in the M1921 and 1928 guns. That is why the M1 and M1A1 have heavier rear receivers than the 1921/1928 guns.

3. It is also often reported that soldiers armed with the M1928 TSMG "took out the H block and threw it away". That is not true. If they did that, they had to use some kind of jury rig because without the H block, the gun cannot be cocked; the H block is the connection between the operating handle and the bolt. I did see a report saying that British armorers welded the pieces together or fashioned a piece of scrap metal to keep the pieces together, which would be possible.

Jim

Booner63
February 18, 2014, 09:52 PM
@Nom de Forum
It's not really a specific person that determined this, it's common convention that handguns with rounds 9mm parabellum or larger are typically short action, or have some other means of delayed blowback. I mistakenly took this and applied this to sub-guns, with the "logic" that pretty much any closed bolt firearm in general I've seen with a caliber less than 9x19mm (handgun or shoulder fired) is direct blowback, while any closed bolt firearm I've seen in calibers greater than 9x19mm firing from a closed bolt (handguns, submachine guns, rifles) have some form of locked breech, be it short action, long action, various mechanical delays (like roller or lever delayed) or gas operation. Hence the confusion with the aforementioned open bolt SMG's.

Booner63
February 18, 2014, 10:06 PM
@ Jim K

I think I recall hearing that while the blish lock did have an effect, it was overall insignificant. I say this because I also recall that one of the changes between the M1/M1A1 and the previous models was that the recoil springs were weakened to reduce rate of fire. Perhaps this is what caused the frame cracking.

And that API bit is very interesting.

EDIT: Nevermind, the recoil spring reduction occurred between the M1921 and M1928

Nom de Forum
February 18, 2014, 10:29 PM
[QUOTE]...... it's common convention that handguns with rounds 9mm parabellum or larger are typically short action, or have some other means of delayed blowback.

I think you are a little confused about terminology. It is common convention for handguns chambered in 9mm and larger to have a mechanically locked breech action of some kind. Short action refers to the physical length of an action, such as in certain models of bolt action rifles that have both short and long actions.

I mistakenly took this and applied this to sub-guns, with the "logic" that pretty much any closed bolt firearm in general I've seen with a caliber less than 9x19mm (handgun or shoulder fired) is direct blowback, while any closed bolt firearm I've seen in calibers greater than 9x19mm firing from a closed bolt (handguns, submachine guns, rifles) have some form of locked breech, be it short action, long action, various mechanical delays (like roller or lever delayed) or gas operation. Hence the confusion with the aforementioned open bolt SMG's.

No worries, we are all here to learn more from our peers. The internet is great for quick and easy to find information, but books by well respected authors (not most gun mag writers) are more reliable for accurate information on fundamental firearms principles.

Booner63
February 18, 2014, 10:38 PM
I think you are a little confused about terminology. It is common convention for handguns chambered in 9mm and larger to have a mechanically locked breech action of some kind. Short action refers to the physical length of an action, such as in certain models of bolt action rifles that have both short and long actions.

My mistake. I was referring to short recoil mechanisms, not short action.

Sam1911
February 18, 2014, 10:47 PM
So Jim, can you address my question from above?

So I've read. I'm still a little unsure of when in the motion of the case and bolt this happens, especially as it is after the bolt picks up the cartridge (otherwise the round would go off in the mag) and after that point the cartridge and bolt are moving at the same speed, until the cartridge stops in the chamber, just a millisecond before the bolt does.

When does the firing pin nub actually IMPACT the primer, and how, if that happens before the case has come to a stop in the chamber?

When/how does the bolt hit the cartridge hard enough to set off the primer, while the bolt and cartridge are traveling at the same rate of speed just BEFORE entering the chamber?

AlexanderA
February 18, 2014, 11:50 PM
I did see a report saying that British armorers welded the pieces together or fashioned a piece of scrap metal to keep the pieces together, which would be possible.

They ground the "ears" (the parts that ride in the receiver slots) off the Blish locks. As you point out, the rest of the Blish lock is needed to link the bolt and the actuator. These modified Blish locks occasionally turn up on the collector market today.

I find the often-repeated claim that British soldiers in North Africa removed the Blish locks from their Thompsons and "pitched them at the Jerries" to be hilarious. If they did that, they would have non-working guns. Even grinding the ears off was a poor practice, since it would increase the cyclic rate and cause stresses that could crack the rear of the receiver. If sand in the actions was a problem, the answer was regular cleaning and maintenance.

Jim K
February 18, 2014, 11:55 PM
OK, first the bolt has a counter sunk area that envelops the case head. In the center of that area is a fixed firing pin usually made as part of the bolt itself.* The firing pin sticks far enough out to fire the primer as the bolt closes. In other words, the open-bolt SMG is a true slam fire; it always goes off before the bolt goes into battery.

But the gun is set up so that as the bolt pushes the round into the chamber ahead of it, the round stops on the chamber shoulder** just an instant before the firing pin slams into the primer. At that point, the bolt is still a millimeter or so out of battery and still has enough forward momentum that the chamber pressure has to stop that momentum before it can begin to push the bolt back. That takes enough force and time that the bullet is out of the barrel and the pressure drops before the bolt can move back far enough to leave the case unsupported.

If that "advanced primer ignition" does not take place, the full force of the chamber pressure has to be resisted by the inert mass of the bolt and the strength of its return spring and one, or usually both, have to be increased.

(As in all blowbacks, the extractor on an SMG has no function in normal operation. It is used only to remove a dud round from the chamber and to act as a pivot point for the ejector.)

*The 21/28 TSMG has a rocker actuated firing pin and the German MP.38/40 has a separate firing pin, but those details are not really important.

**Almost all open bolt SMGs use rimless rounds supported on the case mouth; there have been a few made for rimmed cartridges, mainly .22 LR, but I can't recall any used in WWII as military weapons.

Jim

Jim K
February 19, 2014, 12:06 AM
Hi, AlexanderA,

I have heard several versions of what they actually did, but the "they threw the blocks away" story has been repeated so often that it seems to be accepted by just about every Thompson "expert" who has never seen or used a Thompson. Another common myth (though with some backing) is that all Thompsons could use drum magazines; of course only the 21/28 could do so (though there were some cobbled up drums with tops of stick magazines welded on, so that story is not really false except in regard to issue magazines).

Another is that all TSMG drums held 100 (or more) rounds. There were 100 round drums sold commercially, but the US military issued only 50 round drums and 30 round sticks. The British issued 20 round sticks and 50 round drums.

The designation of the magazines indicated their capacity, in Roman numbers: XX, XXX, L, and C.

Jim

stressed
February 19, 2014, 12:13 AM
Good info in here I never knew. The carbine subgun conversions I have are closed bolt, and would use the mass of the bolt and recoil spring. However there is no real difference from a hot "+P+" round and a standard round when firing, pershaps due to the strong recoil springs, bolt mass and weight. The closed bolt conversions are what foul half the casing the most.

Nom de Forum
February 19, 2014, 12:17 AM
So Jim, can you address my question from above?



When/how does the bolt hit the cartridge hard enough to set off the primer, while the bolt and cartridge are traveling at the same rate of speed just BEFORE entering the chamber?

Here is how Jane's Infantry Weapons describes it: "In such weapons the fixed firing pin of the simple blowback design is replaced by a controlled pin which strikes the cap at the desired point in the forward travel of the cartridge."

Some SMGs and HMGs operate this way.

However, in a somewhat contradictory description of the operation of the advanced primer ignition for the blowback operated Steyr MPi 9mm SMG, Jane's notes that "The breech-block has a fixed firing pin on the bolt face...." Later it goes on to state ".... it can fire as soon as the dimensions of the case and chamber produce sufficient friction for the primer to be crushed by the firing pin." " In general, firing occurs with the round and breech-block still moving forward and about 1.5mm clearance between the front face of the block and the chamber face." The round completely chambers but not when the bolt face meets the chamber face. Pretty tricky stuff for an SMG and easy to screw-up during manufacture.

Tommygunn
February 19, 2014, 12:20 AM
From what I hear the blish lock was ineffective in practice, due to the limited adhesion of the bolt to the bolt face. Brilliant concept though.


The Blish device was initially believed to operate due to a priciple of differential friction. That is essentially that two different metals sliding against each other will cause more friction than identical metals. The Thompson was designed in WW1 and the idea for the delayed blowback device (which also held the bolt assembly together) came from the gun breeches of the big guns on destroyers. The developers tried to scale it down to a rifle size gun, initially in .30-'06. When they discovered they couldn't make it work with the rifle round instead of cominbg up with a workable mechanism, they decided to try it on pistol rounds. It is said John Tafaglio Thompson for some reason had some inordinate obsession with using the Blish Lock. If he hadn't he might have wound up with a full rifle caliber Thompson.
The device works by sliding up and down slightly inside the bolt at an angle, and there are two "ears" on the device that fit into angled slots on the inside of the receiver. These are cut at different angles than the angle on the bolt.
Very recently it was discovered that this device actually does work but not because of friction; those two angles I mentioned; they create a lever-effect and that is what delays the blowback. The 1928 Thompson ran at about 800 rounds per minute, amputate those two "ears" that fit into the slots will result in a Blish that only serves to keep the actuator attached -- a necessity. The result is a Thompson that fires nearer to 1000 RPM.
The M1 Thompson eliminated the Blish device but maintained the firing pin, hammer & firing pin spring of its predecessor. The M1A eliminated the hammer and firing pin & spring.
The face of the bolt is countersunk into the front of the bolt and a milled nib is what hits the primer. The bolt pushes the round forward but it is basically pushing against the top part of the round so that countersunk face isn't directly in contact. The round is pushed up a feedramp where it then aligns with the chamber, and at that point will fit into the countersunk portion of the boltface where the last tiny fraction of an inch forward as the bolt closes will cause that "nib" to ignite the primer.
The M1 and M1A fired at a slightly faster rate than the earlier 1928 model, but by using a recoil spring adjusted for the different weight bolt and making the bolt heavier it still remained inside acceptable army standards.

BOLT from a 1928 Thompson showing the brass colored Blish "ears" and the angle cut in bolt:

Sam1911
February 19, 2014, 07:33 AM
But the gun is set up so that as the bolt pushes the round into the chamber ahead of it, the round stops on the chamber shoulder** just an instant before the firing pin slams into the primer. At that point, the bolt is still a millimeter or so out of battery and still has enough forward momentum that the chamber pressure has to stop that momentum before it can begin to push the bolt back. That takes enough force and time that the bullet is out of the barrel and the pressure drops before the bolt can move back far enough to leave the case unsupported.

Jim,

I nearly accept this, except that the primer wouldn't fire at the point that the firing pin first touches it -- with the bolt the length of the firing pin nub away from being closed -- but instead, it would fire at some undetermined point after the nub/bolt has traveled in even farther, actually crushing the primer.

We know what happens if firearms have insufficient firing pin projection, so just dimpling the primer is not enough. The cartridge won't fire until the pin has inserted itself pretty substantially into the primer. And that doesn't happen until the bolt is just about completely closed.

So it appears to me that whatever degree to which this "API" effect functions in a fixed-firing-pin open bolt sub gun, it must be very, very small.

If that firing pin nub is only about 1-1.5mm long, the primer can't possibly light until the bolt is somewhere between 0.5 and 0.0mm away from being completely closed, and with the speed at which the bolt is moving, I don't think I believe that there's enough time to light off the powder charge and produce case pressure back against the bolt before it is fully closed, that last 0.5 (or less) millimeters.

(A design with a separate firing pin and hammer, or striker, would work very differently, of course, and could be made to fire before the bolt is in battery. I just don't see how a fixed-firing-pin design can.)

Nom de Forum
February 19, 2014, 10:11 AM
Jim K,

This API stuff is a real head scratcher concept with a fixed firing pin. Is API really making a difference in a fixed firing pin SMG that it is worth the trouble of designing to use it? Is bolt weight really reduced significantly, because the time/distance for bolt inertia to have an effect is very short by the time power ignition occurs? Jane's refers to a as much as a 50% reduction in bolt weight, is that really possible?

Jim K - ..... the round stops on the chamber shoulder** just an instant before the firing pin slams into the primer. At that point, the bolt is still a millimeter or so out of battery and still has enough forward momentum that the chamber pressure has to stop that momentum before it can begin to push the bolt back. That takes enough force and time that the bullet is out of the barrel and the pressure drops before the bolt can move back far enough to leave the case unsupported.

Does this mean that the if the bolt manually very slowly closed its face would be held away from the chamber face by the firing pin contacting the uncrushed primer?

Nom de Forum -".... it can fire as soon as the dimensions of the case and chamber produce sufficient friction for the primer to be crushed by the firing pin." " In general, firing occurs with the round and breech-block still moving forward and about 1.5mm clearance between the front face of the block and the chamber face."

Could this mean a tapered chamber that the final closing on the bolt force the casing into just before power ignition?

Sam1911 - If that firing pin nub is only about 1-1.5mm long, the primer can't possibly light until the bolt is somewhere between 0.5 and 0.0mm away from being completely closed, and with the speed at which the bolt is moving, I don't think I believe that there's enough time to light off the powder charge and produce case pressure back against the bolt before it is fully closed, that last 0.5 (or less) millimeters.

Full ignition powder is obviously measured in milliseconds, but I also wonder about the time available to make API really of value in a fixed firing pin design.

Sam1911
February 19, 2014, 10:37 AM
Does this mean that the if the bolt manually very slowly closed its face would be held away from the chamber face by the firing pin contacting the uncrushed primer?
Yes, and that's what prompts my disbelief in API as a function of an open bolt gun.

1) The only time the bolt/firing-pin can fire the cartridge is after the cartridge has stopped moving. It doesn't stop moving until it is against the chamber shoulder, fully inside the chamber.

2) When the cartridge stops moving, the tip of the firing pin is against the face of the primer. The only distance left for the bolt to travel is the length of the firing pin nub, which is quite short.

3) The primer does not light when the firing pin arrives at the face of the primer, or even when it has dimpled the primer, but only when some significant protrusion into the primer has occurred.

4) Whatever amount of protrusion into the primer is required to get it to light is linear distance traveled in the bolt's forward motion. Therefore the bolt is even closer to completely closed by the time that primer ignites.

5) And then some tiny bit of time still has to pass for the primer to flash and ignite the powder, and the powder has to combust and begin to expand, and the case has to overcome its inertia and start to move backward against the bolt. Not much time for all of those things to happen, but still time during which the bolt is still moving forward.

So my view of it is that the bolt must have exceedingly little, if any, forward travel left by the time the case starts to push back against it.

rcmodel
February 19, 2014, 12:22 PM
The Oerlikon 20 mm cannon is an example of API blow-back operation used in a big gun.

http://en.wikipedia.org/wiki/Oerlikon_20_mm_cannon

rc

Jim K
February 19, 2014, 04:03 PM
The idea that a firing pin needs to penetrate deeply into a primer comes from looking at fired primers where primer metal extrusion results in the appearance of a deep penetration. Deaden a primer and try to "fire" it in your favorite handgun, and you will see that the "deep penetration" doesn't exist and isn't needed to fire the primer.

Sorry, but API does exist, and it is used on all those open bolt guns, even the .22 rifles made that way to reduce bolt mass.

It is very hard for us to think in the small times involved in firing a gun, but we have to realize that primer ignition, powder ignition and pressure buildup occur in fractions of a millisecond.

Jim

Sam1911
February 19, 2014, 04:23 PM
Sorry, but API does exist,

Ok, Jim, no need to be sorry. But light primer strikes happen with a decent amount of frequency, too, so SOME amount of primer indentation is required, and not just a little. And even so the primer does not light off when the firing pin simply arrives AT the face of the primer.

So, please explain the sequence to me of how this works in an open-bolt, fixed firing pin gun.

1) Bolt strips cartridge
2) Bolt takes cartridge to chamber
3) Cartridge enters the chamber and stops
4) Bolt moves forward, let's say 1/2 of the length of the firing pin nub before the primer ignites? That's far more than fair.
5) If normal firing pin protrusion is something like 0.06", then that would be 0.03" of travel left to go before the bolt is fully closed.
6) Doing a little rough math, that means the bolt will have about one millisecond left to travel once the primer ignites.
7) How fast does the primer light, the powder start to burn, and the case start to push back? Less than one millisecond? Substantively so?

Sam1911
February 19, 2014, 04:25 PM
In a striker-fired or hammer-fired gun, the mechanism can be made to actually strike the firing pin before the cartridge arrives at the chamber, or while it is not yet COMPLETELY seated in the chamber.

In a fixed-pin design, that cannot possibly happen. So please continue to explain.

Jim K
February 19, 2014, 04:32 PM
Sorry, Sam, I just cannot seem to explain in any way that will convince you, so I don't see any point in continuing. Maybe we can discuss the number of angels that can dance on the head of a firing pin. ;)

Jim

Booner63
February 19, 2014, 05:09 PM
Here's my thinking on open bolt, fixed firing pin API: It's not true API, but it produces effects similar to API.
In "traditional" API, the cartridge is ignited by a mobile firing pin while the bolt is still moving forward. As Sam1911 makes clear, the cartridge could not fire while the bolt is moving forward, so it is not technically API.

But what about momentum? The forward momentum of the firing pin can continue, even if the forward motion does not. So at the point of ignition, the bolt is not able to move forward, but it still has forward inertia, causing a slight force for the cartridge to overcome that is not found in closed bolts. Not sure if this would account for a 50% bolt mass reduction but hey.

Nom de Forum
February 19, 2014, 05:31 PM
Sorry, Sam, I just cannot seem to explain in any way that will convince you, so I don't see any point in continuing. Maybe we can discuss the number of angels that can dance on the head of a firing pin. ;)

Jim

O.K., I am no longer scratching my head about this one. It has been 25 years since I spent much time shooting, repairing, thinking about SMGs so the whole API thing just faded away. After reading your posts Jim and doing a little more reading in some old books the fog is clearing. I feel a little embarrassed :o after just participating in another thread about the duration in milliseconds of locktime to have not thought clearly about the milliseconds after locktime. No need to discuss how many angels can dance on the head of a firing pin, it is about how quickly and long the gas pressure dances on the end of the firing pin/bolt face. The very small amount of primer deformation for ignition makes it understandable. What amazes me is just how short in time and distance the API effects can be useful in a fixed firing pin firearm. I did find evidence it can be problematic if the chamber fouling causes early ignition so SMG bolt heads are designed to safely handle this situation. Thanks Jim for your patience in explaining long enough for me to reboot my mind.

Sam1911
February 19, 2014, 06:10 PM
Well, it's just awesome that you two have it sorted out! Would you please explain it to me?

It isn't religion or sub-atomic physics. There isn't any element of belief required for this to work. :) It either is a physical phenomenon that can be measured or it is not. Don't tell me it's angels and pinheads. Do better than that.

So what I'm asking one or both of you to do is look at my firing sequence given above and pick it apart. Show me the step that I've misunderstood or oversimplified.

I assume it has to be somewhere between steps 5 and 6 that you maintain this effect takes place. Is that correct? Is my timing calculation off? Is it really a matter of an action taking a measurable effect in the (what appears to me to be) 1 millisecond between the primer igniting and the bolt coming to a stop against the barrel?

Walk me through where we agree, and then maybe we can better pick apart what I'm not understanding.

Sam1911
February 19, 2014, 06:15 PM
So at the point of ignition, the bolt is not able to move forward, but it still has forward inertia, causing a slight force for the cartridge to overcome that is not found in closed bolts. Not sure if this would account for a 50% bolt mass reduction but hey.
Ok, what? It's been a long time since college physics, but the phrase "forward inertia" would seem to REQUIRE motion, would it not? And if the bolt has come to a stop, it has inertia, but the inertia of an object at rest, not one in motion.

All closed bolt blowback guns would have the same inertia then.


...Unless you're saying that it has slammed into the rest of the gun causing the whole gun to move TOGETHER and so it isn't truly at rest?

Nom de Forum
February 19, 2014, 07:18 PM
Well, it's just awesome that you two have it sorted out! Would you please explain it to me?

It isn't religion or sub-atomic physics. There isn't any element of belief required for this to work. :) It either is a physical phenomenon that can be measured or it is not. Don't tell me it's angels and pinheads. Do better than that.

So what I'm asking one or both of you to do is look at my firing sequence given above and pick it apart. Show me the step that I've misunderstood or oversimplified.

I assume it has to be somewhere between steps 5 and 6 that you maintain this effect takes place. Is that correct? Is my timing calculation off? Is it really a matter of an action taking a measurable effect in the (what appears to me to be) 1 millisecond between the primer igniting and the bolt coming to a stop against the barrel?

Walk me through where we agree, and then maybe we can better pick apart what I'm not understanding.

I will try but don't expect this to be a good explanation of something I think I marginally understand and have little confidence in explaining why.

The distance the firing pin travels into the primer is sufficient for detonation and to allow enough movement of the bolt face for API to have an effect. As short as the time the bolt face has before coming in contact with the chamber face it is longer than the time it takes the primer to detonate, powder to ignite, and gas volume to build sufficiently to begin exerting pressure through the case onto the bolt face . We are talking about not milliseconds but fractions of a millisecond for this to occur and it is faster than the remaining time of movement of the bolt face toward the chamber face. Take a look at this link. It has a graph near the bottom that shows time between ignition and the bullet exiting the barrel.

http://www.frfrogspad.com/intballi.htm

Sam1911
February 19, 2014, 07:30 PM
Ok, that is very helpful! I wonder if anyone has a chart anywhere exploring bolt velocity and travel distance the same way? Almost certainly no, or not easily available, but it sure would be very useful to see. My 1 millisecond estimate was literally worked up in 30 seconds on a scratch pad at work, so I doubt it's very close to right.

However, we are then agreed that there is no pre-ignition before the cartridge is seated, and that whatever the "API-like" effects are, they must happen over the course of the last 0.03" or less of bolt movement.

rcmodel
February 19, 2014, 08:37 PM
Try to think of a 'very slightly out of battery, controlled slam-fire'.

That's the best simple explanation of API I can come up with.

And yes, it exists, and yes, it is designed into the gun from the get-go.

And also yes, The Oerlikon 20 mm cannon was real, and could not have possibly worked the way it does without API.

rc

Sam1911
February 19, 2014, 08:46 PM
I understand all of that, what I'm trying to figure out is the what/how/when of the forces involved. Seems like the effect hovers somewhere between "bolt 100% closed" and "bolt 99.999% closed."

SlamFire1
February 19, 2014, 09:58 PM
Excellent descriptions of the principles of blowback and all automatic mechanism can be read in volume IV of The Machine Gun by George M. Chinn. Even though this series have been out there for almost 60 years, hardly anyone reads Volume IV. Few people read Vol 1 either, but of those who read any of Chinn’s books, most people prefer the pretty pictures and history found in Vol 1 compared to the operating principals in Vol IV. You can download all of the volumes for free here:
http://www.milsurps.com/content.php?r=347-The-Machine-Gun-(by-George-M.-Chinn)

Not only was the Oerlikon 20 mm cannon for real, it used greased ammunition! You can see at exactly 2:14 on this WW2 video a Sailor’s hand painting grease on the 20 mm ammunition loading machine for the Oerlikon anti aircraft machine guns.

http://www.youtube.com/watch_popup?v=9dR3h2HdnBQ

This is a direct contradiction to the section in Hatcher’s Notebook which claims that greased ammunition dangerously raises pressures . That whole section is built around junk science, junk science used to excuse the Army of responsibility when single heat treat 03’s blew up on the firing line, and a fortuitously timed cover up when the tin can ammunition of the 1921 National Matches started blowing up rifles. The 1921 National Match ammunition was issued with tinned bullets. The tin cold soldiered itself to the case necks and that created a horrible bore obstruction which blew up rifles. The fact that the Oerlikon was firing around 700 to 900 greased rounds a minute should cause any intelligent being to question the objectivity of Hatcher’s account and the “research” proving that grease was the problem.

As Chinn says:

http://img.photobucket.com/albums/v479/SlamFire/Reloading/Case%20Lubrication/ChinnBlowbackLubricatedcasesOilomatic_zpsc04fe442.jpg (http://smg.photobucket.com/user/SlamFire/media/Reloading/Case%20Lubrication/ChinnBlowbackLubricatedcasesOilomatic_zpsc04fe442.jpg.html)

I have not looked inside my Chinn for technical data tonight, his books are concept books. But I have found a dribble of data in Brassey’s “Small Arms” by D. F Allsop & M.A. Toomey. This is an excellent book, too bad even fewer people have read it than Chinn’s series.

In this book, the section dealing with blow back with Advanced Primer Ignition starts on page 75. It does discuss the British 9mm SMG L2A3 and its API. In so far as your question, here is the snippet: “As a broad approximation the bolt has some 0.76 mm to go before it encounters the breech face. The impulse given by the burning of the propellant causes a rapid slowing up the forward movement of the breech block. When the maximum chamber pressure has developed as shown in Fig 6.3, the breech block is still 0.46mm clear of the rear face of the chamber and still moving forward. Thus while the bullet is still in the bore the breech block is either moving forward, momentarily at rest, or being driving slowly backwards.”

rcmodel
February 19, 2014, 10:25 PM
Yep, that's it all right!

Thanks for the memory jogger!

Rc

Booner63
February 19, 2014, 10:31 PM
@Sam1911
Ok, what? It's been a long time since college physics, but the phrase "forward inertia" would seem to REQUIRE motion, would it not? And if the bolt has come to a stop, it has inertia, but the inertia of an object at rest, not one in motion.

The difference that this would make is most likely pretty minor, but I figured I'd say it since it does occur in open bolt guns, but not in closed bolts (at least not with the bolt anyway; maybe with other firing mechanisms)

Whenever something is pushed, it doesn't behave like it appears to: like a rigid body moving with all portions at the same rate. Instead, it's atoms slowly displace each-other, moving at the speed of sound (this was a theory of how the speed of light could be topped: you have a point one light year away from a starting point; at this a plank of wood stretches from the start to nearly the end. The idea is that if light is shined from this point at exactly the same time the board is shoved, the board would reach the end-point first. But this is not so. Through observing the behavior of other matter it was discovered that if we could perform an experiment like this, the board would appear to compress at the beginning, and after a long time of atoms displacing each-other, would appear to un-compress at the endpoint. However, it would take the board 891,837 years to do so moving at the speed of sound, while it would take light only one year)

So if we apply this to what happens in an open bolt mechanism, once the bolt face strikes the breech face, the bolt would "compress" as the cartridge started to move backwards, meaning the cartridge would have to overcome the forward motion of the rearward molecules of the bolt as well as compress the frontwards end of the bolt,whereas in a closed bolt the rearwards end has already "compressed" and "uncompressed", and the cartridge only has to compress the front end.
Again, the effect this would have is most likely minor, as the length of the bolt relative to the distance sound (and thus the molecules) travel in the time it takes for the cartridge to ignite is practically immeasurable. But thought I might as well throw that out there.

Nom de Forum
February 20, 2014, 12:13 AM
Excellent descriptions of the principles of blowback and all automatic mechanism can be read in volume IV of The Machine Gun by George M. Chinn. Even though this series have been out there for almost 60 years, hardly anyone reads Volume IV. Few people read Vol 1 either, but of those who read any of Chinn’s books, most people prefer the pretty pictures and history found in Vol 1 compared to the operating principals in Vol IV. You can download all of the volumes for free here:
http://www.milsurps.com/content.php?r=347-The-Machine-Gun-(by-George-M.-Chinn)

Not only was the Oerlikon 20 mm cannon for real, it used greased ammunition! You can see at exactly 2:14 on this WW2 video a Sailor’s hand painting grease on the 20 mm ammunition loading machine for the Oerlikon anti aircraft machine guns.

http://www.youtube.com/watch_popup?v=9dR3h2HdnBQ

This is a direct contradiction to the section in Hatcher’s Notebook which claims that greased ammunition dangerously raises pressures . That whole section is built around junk science, junk science used to excuse the Army of responsibility when single heat treat 03’s blew up on the firing line, and a fortuitously timed cover up when the tin can ammunition of the 1921 National Matches started blowing up rifles. The 1921 National Match ammunition was issued with tinned bullets. The tin cold soldiered itself to the case necks and that created a horrible bore obstruction which blew up rifles. The fact that the Oerlikon was firing around 700 to 900 greased rounds a minute should cause any intelligent being to question the objectivity of Hatcher’s account and the “research” proving that grease was the problem.

As Chinn says:

http://img.photobucket.com/albums/v479/SlamFire/Reloading/Case%20Lubrication/ChinnBlowbackLubricatedcasesOilomatic_zpsc04fe442.jpg (http://smg.photobucket.com/user/SlamFire/media/Reloading/Case%20Lubrication/ChinnBlowbackLubricatedcasesOilomatic_zpsc04fe442.jpg.html)

I have not looked inside my Chinn for technical data tonight, his books are concept books. But I have found a dribble of data in Brassey’s “Small Arms” by D. F Allsop & M.A. Toomey. This is an excellent book, too bad even fewer people have read it than Chinn’s series.

In this book, the section dealing with blow back with Advanced Primer Ignition starts on page 75. It does discuss the British 9mm SMG L2A3 and its API. In so far as your question, here is the snippet: “As a broad approximation the bolt has some 0.76 mm to go before it encounters the breech face. The impulse given by the burning of the propellant causes a rapid slowing up the forward movement of the breech block. When the maximum chamber pressure has developed as shown in Fig 6.3, the breech block is still 0.46mm clear of the rear face of the chamber and still moving forward. Thus while the bullet is still in the bore the breech block is either moving forward, momentarily at rest, or being driving slowly backwards.”
SlamFire1,

I am not so sure Chinn's statement about the need for lubrications is appropriate for all types of blowback action firing "high powered ammunition". I will have to check but I don't think some of the autocannons using blowback with lock actions require it. I do know that oiling cartridges has been a desperate measure to improve functioning in other poorly designed automatic weapons. Thanks for the link to Chinn's books. They were always too expensive when I was young and poor.

caribou
February 20, 2014, 05:00 AM
Oiling or waxing full sized rifle cartridges was common until it was discovered that 'pre extraction' was needed, often in the form of a twist or slight pull on a case that's been fired as the mechanism opens. The problems of increased pressures was found that excessive oiling in the chamber would lead to "dieseling' , igniting the oils under pressure, creating higher pressures. The smaller the surface area, the higher the pressure. Larger chambers have larger surface areas and barrels around them able to withstand the dieseling effect much more easily.
Other ideas were used to solve primary extraction problems. One of the most effective ways to extract a cartridge that was still under residual pressure and chamber adhesion was fluting.
Fluting a straight walled cartridge chamber would help with primary extraction, with gas floating the case, but rotation of the unlocking bolt is the most common was to deal with high pressure case extraction in modern small arms.

The 9X18 Makarov round was developed from the German "Ultra" 9mm round, which was as high a performance that a blowback Pistol was tolerable in weight and safety with, not a submachine gun. Subguns can burn some pretty hot ammo, which would make a pistol rather heavy, needing a locking mass and the associated weight with a full sized pistol. Haveing an effective round and a simple, light, unlocked mechanism called for pushing the limits for pure blowback. 9x18 was the sweet spot in pistols that work.

H&K came up with a relatively light submachine gun that started its 'fireing cycle' with the bolt closed, and without a bolt flopping forth to upset precise aim and ignite the cartridge, the MP-5 is light and accurate while being safe with very high velocity rounds.

SlamFire1
February 20, 2014, 08:09 AM
SlamFire1,

I am not so sure Chinn's statement about the need for lubrications is appropriate for all types of blowback action firing "high powered ammunition". I will have to check but I don't think some of the autocannons using blowback with lock actions require it. I do know that oiling cartridges has been a desperate measure to improve functioning in other poorly designed automatic weapons. Thanks for the link to Chinn's books.

Greases and oilers basically went on the ash heap of history after chamber flutes were introduced by the Russians, copied by the Germans and used in their roller bolts. I think chamber flutes are a better solution, but even today, FN decided to go the route of a lubricated cartridge in 5.7 cartridge.

FN PS90 5.7×28mm

http://en.wikipedia.org/wiki/FN_5.7%C3%9728mm

FN's 5.7×28mm cartridge cases are covered with a special polymer coating for easier extraction with the PS90 carbine due to the high chamber pressures and lack of case tapering.[32] In addition, this coating ensures proper feeding and function in the magazines.[32]

Paul Mauser used greased 8mm cartridges in his early 1905 semi automatic rifle, but greases and oils are undesirable from many respects for field weapons. It is my recollection that the pre WW2 requirements the US Army put out for a semi automatic rifle were for a mechanism that required no oilers. Some of the earliest semi rifles, like the Thompson, has oiled pads in the magazines, I suspect these were dirt magnets .

They were always too expensive when I was young and poor.

I paid $100.00 for my Vol 4 and was happy to pay it, because those Chinn books were hard to find. Hatcher's Notebook was less than $20.00 and has gone through at least 15 editions. And there in lies the problem, after Hatcher's Notebook comes out , with that section on the "dangers" of greased bullets, it is as if the entirety of the American shooting community took a drill to their forehead and sucked out half their brains through the hole. There are other cases of group amnesia as large (weapons of mass destruction anyone?) but this one totally removed the memory of greased cases, oiled cases, oilers, etc, from the shooting community, with Hatcherites shouting down anyone who brought up the practice.


The problems of increased pressures was found that excessive oiling in the chamber would lead to "dieseling' , igniting the oils under pressure, creating higher pressures. The smaller the surface area, the higher the pressure. Larger chambers have larger surface areas and barrels around them able to withstand the dieseling effect much more easily.

During load development I lubricate the heck out of my cartridges because I do not want to reduce bolt thrust or to confuse the transition from rounded to flat primers. I want the full bolt thrust applied to my bolt lugs so I can tell just when I have a maximum load. Parasitic friction between a dry case and a dry chamber disguises maximums loads and it is common to read statements by shooters who get a drop of oil on their cartridges and suddenly experience high pressure indications. The problem for them was that their reloads were over max, but case to chamber friction hid that fact, and once that friction is broken, they see high pressure indications.

In so far as dieseling, I have never seen it in 223, 257 Roberts, 6.5 X 55, 270 Win, 308, 30-06, etc, and virtually every WW1 service cartridge was used in a mechanism with an oiler. If dieseling was a problem I would have seen it and read about it long ago.

BobWright
February 20, 2014, 10:15 AM
The famous "Grease Gun", or M3 submachine gun, was indeed simple blowback. In fact, it was simplicity to the utmost.

Original M3s had a cocking lever on the side of the receiver, the M3A1 simply had a hole in the bolt. The shooter opened the cover of the ejection port, stuck his index finger into the hole, and pulled the bolt back. The gun could be cocked by opening the cover and swinging the gun downward smartly and jerking it to a stop. Inertia would cock the bolt.

The cartridge case was stopped by the chamber before the firing pin struck and fired the primer.

The M3A1 could be changed from .45 ACP to 9mm by unscrewing the barrel, removing the bolt assembly, replacing the bolt assemby with a correct 9mm assembly, and screwing a 9mm barrel onto the receiver. New magazines were required, of course.

The cyclic rate of fire was 450 RPM, and made a pop-pop-pop sound. Removing the bolt from the guides and threading two extra recoil springs onto the guide bars greatly increased the rate of fire. The sound was more of a brrrrp!. Don't know what the resulting rate of fire was, but sure sounded impressive. Resulted in some jams, though.


Bob Wright

Arizona_Mike
February 20, 2014, 03:03 PM
As a rule of thumb, 9mm Mak is about the max you can run in a conventional pistol and have reasonable weight and slide racking force. It is a rough technical limitation for that application. Larger guns can use higher reciprocating mass (the biggest effect in delaying a blowback) and spring pre-tension (lesser impact) and a side (or top) handle to give the human operator a better grip than pinching a slide.

MechTech pistol-carbine upper receivers have a BCG (they call it something different) that weights about 2.5lb and high spring pre-tension and work great with 10mm Auto and .45 Rowland. The reciprocating part is almost half the weight of the assembled upper+lower. My .45 and 10mm uppers shoot very clean, cleaner than a recoil operated pistol.

Mike

Telekinesis
February 20, 2014, 03:53 PM
SlamFire, thanks for the link to Chinn's The Machine Gun. I've been looking everywhere for an affordable print version, but being able to have all the volumes in PDF is great.

Gonna be doing some light reading when I get home tonight :D

mrming
February 20, 2014, 03:55 PM
http://www.orions-hammer.com/blowback/

That fellow seems to have worked out the majority of the math. Its mostly based on the aforementioned Chin book.

9mm bolt should end up weighing around 1.8 pounds as I recall if you are relying upon inertia alone.

http://www.uzitalk.com/forums/archive/index.php/t-1389.html

Uzi, which I think most agree is an open bolt design come in around 1.4 or less. so, clearly additional forces are in play.

Dwell time for pressure curve from the first link is down around 1 ms.

I recall but am unable to provide links at present for bolt action strength calcs. While friction of the case in chamber wasn't a dominate factor it was significant enough to be worth calculating.

i'd suggest that it is a thing, and it seems observable in existing arms by reviewing bolt weights in some modern pieces vs the expected weights.

If we assume the bolt is moving in recoil at around 12 fps, spring probably isn't bouncing it back in any faster. Most likely, its moving a lot slower.

12 fps * 12 inch = 144. 144 / 1000 (ms) = .144 inch per ms. A mm is roughly 0.03". Is it reasonable to expect a 5 pound recoil spring to create a return velocity of 1/10th or less vs the 1k+ pound impulse applied the opposite way by the cartridge firing? Probably.

The time it takes in forward travel of that 1mm of protrusion of the firing pin is probably longer than the entire dwell time required for the pressure to discharge.

Only question left is.. how fast do primers ignite? That I don't have a ready answer for. Most explosives have a detonation velocity well above 5k fps. How long for it to mechanically light though? No clue. But i'd be surprised if the primer ignition takes longer than the entire powder burn and pressure drop off. Given the forward motion being say, 1/4 of the rear it'd give a full ms for ignition before fully seating the bolt on just a 1mm protrusion.

Nom de Forum
February 20, 2014, 05:00 PM
[QUOTE]Greases and oilers basically went on the ash heap of history after chamber flutes were introduced by the Russians, copied by the Germans and used in their roller bolts. I think chamber flutes are a better solution, but even today, FN decided to go the route of a lubricated cartridge in 5.7 cartridge.

Tony Williams in Flying Guns of WWII writes that the fluted chamber "seems to have been an Italian invention". Perhaps it was, but are you referring to the Russians as being the first to actually field a fluted chamber gun?

Paul Mauser used greased 8mm cartridges in his early 1905 semi automatic rifle, but greases and oils are undesirable from many respects for field weapons. It is my recollection that the pre WW2 requirements the US Army put out for a semi automatic rifle were for a mechanism that required no oilers. Some of the earliest semi rifles, like the Thompson, has oiled pads in the magazines, I suspect these were dirt magnets .

Need for oiled cartridges in a infantry weapon is certainly not desirable. They certainly hindered, an already hindered by mostly outdated weapons, Imperial Japanese Army by being required for their Hotchkiss derivative MGs.

Regarding oiled cases in autocannons: according to Tony Williams the British were able to get the Hispano-Suiza HS404 to work without oiling by altering the case design, but mentions "no record has been found of how this was achieved. As you probably know the HS404 and derivatives were the most used Allied aircraft cannon type. Have any ideas about this?

I paid $100.00 for my Vol 4 and was happy to pay it, because those Chinn books were hard to find. Hatcher's Notebook was less than $20.00 and has gone through at least 15 editions. And there in lies the problem, after Hatcher's Notebook comes out , with that section on the "dangers" of greased bullets, it is as if the entirety of the American shooting community took a drill to their forehead and sucked out half their brains through the hole.
There are other cases of group amnesia as large (weapons of mass destruction anyone?) but this one totally removed the memory of greased cases, oiled cases, oilers, etc, from the shooting community, with Hatcherites shouting down anyone who brought up the practice.

The $100 you paid is what I remember the price was in 1979, close to half the cost of a new Colt Series 70.

Your comments on Julian Hatcher are interesting and the situation is not unusual. Someone gets a reputation as a Guru, and faithful develop a blind spot in their critical thinking skills and trust without verification. Over the last forty years I have noticed the same with the Cooperites. Chairman Jeff often got the same pass as other heavily credentialed authorities and it was compounded by his affected charismatic mannerisms. It is just human nature to avoid looking the at the clay feet of our idols.

Nom de Forum
February 20, 2014, 05:06 PM
http://www.orions-hammer.com/blowback/

That fellow seems to have worked out the majority of the math. Its mostly based on the aforementioned Chin book.

9mm bolt should end up weighing around 1.8 pounds as I recall if you are relying upon inertia alone.

http://www.uzitalk.com/forums/archive/index.php/t-1389.html

Uzi, which I think most agree is an open bolt design come in around 1.4 or less. so, clearly additional forces are in play.

Dwell time for pressure curve from the first link is down around 1 ms.

I recall but am unable to provide links at present for bolt action strength calcs. While friction of the case in chamber wasn't a dominate factor it was significant enough to be worth calculating.

i'd suggest that it is a thing, and it seems observable in existing arms by reviewing bolt weights in some modern pieces vs the expected weights.

If we assume the bolt is moving in recoil at around 12 fps, spring probably isn't bouncing it back in any faster. Most likely, its moving a lot slower.

12 fps * 12 inch = 144. 144 / 1000 (ms) = .144 inch per ms. A mm is roughly 0.03". Is it reasonable to expect a 5 pound recoil spring to create a return velocity of 1/10th or less vs the 1k+ pound impulse applied the opposite way by the cartridge firing? Probably.

The time it takes in forward travel of that 1mm of protrusion of the firing pin is probably longer than the entire dwell time required for the pressure to discharge.

Only question left is.. how fast do primers ignite? That I don't have a ready answer for. Most explosives have a detonation velocity well above 5k fps. How long for it to mechanically light though? No clue. But i'd be surprised if the primer ignition takes longer than the entire powder burn and pressure drop off. Given the forward motion being say, 1/4 of the rear it'd give a full ms for ignition before fully seating the bolt on just a 1mm protrusion.

I suspect the designers of API blowback SMGs used a significant "fudge factor" back in the slide rule days to increase the bolt weight so that any tolerance problems from manufacturing or wear would be compensated to prevent too many blown cases.

Jim Watson
February 20, 2014, 05:49 PM
The problems of increased pressures was found that excessive oiling in the chamber would lead to "dieseling' , igniting the oils under pressure,

How is the oil to ignite when there is no air? A cartridge is a lean environment, there is not enough oxygen in the powder to oxidize even all the carbon and hydrogen of the nitrocellulose and nitroglycerine.

goon
February 20, 2014, 08:24 PM
How is the oil to ignite when there is no air? A cartridge is a lean environment, there is not enough oxygen in the powder to oxidize even all the carbon and hydrogen of the nitrocellulose and nitroglycerine.

But won't there always be air available to the chamber from the breech and muzzle ends of the barrel? What I'm getting out of this discussion is that the "dieseling" is occurring between the chamber and the outside of the casing. The oil would ignite when compressed between the chamber wall and the outside of the casing and combustion would occur because so much space at the front and rear of the chamber would still allow oxygen to be present.

Admittedly though, I'm a novice in this conversation.

SlamFire1
February 20, 2014, 08:36 PM
Tony Williams in Flying Guns of WWII writes that the fluted chamber "seems to have been an Italian invention". Perhaps it was, but are you referring to the Russians as being the first to actually field a fluted chamber gun?
The book, German Military Rifles and Machine Pistols 1871-1945 by Has Dieter Gotz makes the claim that the Germans captured a Russian machine aircraft machine gun in Spain and it had a fluted chamber. That was a key invention that made the delayed blowback roller bolt function.

I also have the book “Full Circle” and a number of people make the claim they invented the delayed blowback roller bolt, there are probably as many claimants to the invention of the fluted chamber as there are claimants to the Czarist throne


Regarding oiled cases in autocannons: according to Tony Williams the British were able to get the Hispano-Suiza HS404 to work without oiling by altering the case design, but mentions "no record has been found of how this was achieved. As you probably know the HS404 and derivatives were the most used Allied aircraft cannon type. Have any ideas about this?

That would be interesting to know if it was done, but without physical specimens or test reports, I will stick with Chinn. Vol 1 page 571 “The British made available to the United States at this time the results of a test show the failure of an attempt to fire the weapon without lubricating the ammunition.”

Your comments on Julian Hatcher are interesting and the situation is not unusual. Someone gets a reputation as a Guru, and faithful develop a blind spot in their critical thinking skills and trust without verification. Over the last forty years I have noticed the same with the Cooperites. Chairman Jeff often got the same pass as other heavily credentialed authorities and it was compounded by his affected charismatic mannerisms. It is just human nature to avoid looking the at the clay feet of our idols.

The more I study this, the more I find what General Hatcher knew and when he knew it. I cannot believe that he did not understand that what he was writing in 1947 was a whitewash of an Army created train wreck. I have gone through the Arms and Man on Google books, read that Hatcher was there at the beginning of so many things. He was at the meeting in 1920 that decided to get greased bullets out of matches, the primary concerns were about dirt getting on the grease and scratching the chambers/barrels of loaner rifles. But, interestingly enough, the topple point, the final argument against those who wanted to keep grease in, was “increased bolt thrust”. Hatcher knew, as did everyone on that board, that increased bolt thrust is only a concern if you are issuing structurally deficient rifles. And at that time, the Army had one million low number 03’s which were breaking all over the place. But neither he, nor anyone else at a high level in the Ordnance Dept is letting it be known that Army rifles were defective. Hatcher shot high power against Swiss teams in Switzerland, (they used greased bullets till the 1980’s), he was the illustrator for the 1936 Major Naramore book. Naramore points out that the tin on the bullet of the tin can ammunition caused a bore obstruction, therefore the fault was with the ammunition. General Hatcher writes for the 1930’s Army Ordnance magazine, when he lead Small Arms Development, about lubrication being needed for delayed blowback mechanisms. When he was in charge of Army Ordnance, the US built over 150,000 20 mm cannons that required greased ammunition. He was also a reviewer of Chinn’s Machine Gun Book series.

Post WW2, in his retirement, Hatcher needs to maintain good relations with the Army. He is going to use all his personnel connections, management abilities, personnal charm to crawl to the top of the NRA, and that requires not antagonizing the Army. I have examined the current salary structure, his NRA Executive Board income was probably six to eight times greater than his Army retirement. Currently, the top NRA positions are paid $600,000 to $800,000 a year. The NRA of the period is an Army creation, has been since the 1870's, was run during his junior officer years by an active duty Brigadier General, was later run by active duty and retired military officers, and whose operating budget is heavily subsidized by Army money. It is my recollection, from the American Rifleman of the early 60’s, Army money was 25% of the total NRA budget. In everything he wrote, Hatcher minimizes the Army role and responsibility for any Army created failure.

SlamFire1
February 20, 2014, 08:41 PM
How is the oil to ignite when there is no air? A cartridge is a lean environment, there is not enough oxygen in the powder to oxidize even all the carbon and hydrogen of the nitrocellulose and nitroglycerine.

But won't there always be air available to the chamber from the breech and muzzle ends of the barrel? What I'm getting out of this discussion is that the "dieseling" is occurring between the chamber and the outside of the casing. The oil would ignite when compressed between the chamber wall and the outside of the casing and combustion would occur because so much space at the front and rear of the chamber would still allow oxygen to be present.

Admittedly though, I'm a novice in this conversation.

Maybe you are thinking of dieseling in air rifles. In those mechanisms special oils have to be used in the compression chamber or there will be dieseling. Dieseling has not been a concern in rimfire or centerfire mechanisms.

Jim Watson
February 20, 2014, 10:09 PM
There just isn't anywhere in there for enough air to burn anything.
Red herring.

goon
February 20, 2014, 11:00 PM
Maybe you are thinking of dieseling in air rifles. In those mechanisms special oils have to be used in the compression chamber or there will be dieseling. Dieseling has not been a concern in rimfire or centerfire mechanisms.

I'm just a guy on the internet pontificating about stuff I really don't know much about. ;)

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