Direct Blowback in WWII submachine guns

[Booner63]

@ 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]

...... 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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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 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]

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]

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]

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]

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]

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