Direct Blowback in WWII submachine guns

[Booner63]

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).

 

[Sam1911]

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]

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]

@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]

@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]

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]

It was used in the German M38, MP40, MP44, and later on the Suomi M31, and later still on the UZI.

rc

 

[Sam1911]

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]

I thought the MP44 was gas operated?

 

[Sam1911]

Yes.

 

[rcmodel]

Oops!

My typer got ahead of my thinker!

Rc

 

[Booner63]

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]

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]

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]

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]

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]

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]

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]

@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]

Is this found on all of your blowbacks, or only on ones that fire "larger" ammunition?

 

[Sam1911]

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]

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]

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]

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]

@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.