SMGs in ww2 with much smaller cartridges?

The Japanese apparently imported a number of MP 28 and MP 34 SMG's for testing, before designing the Type 100 SMG in 8mm Nambu. For a more powerful cartridge they could just have adopted the 9x19 (which likely was the caliber of the above mentioned imported SMG's), no need to reinvent the wheel.

But, as mentioned, the lack of need per doctrine was a far bigger issue than choosing a non-optimal caliber.

From Historical Firearms - Japanese Submachine Guns Like many of the major... :

The explanation for Japan's apparent lack of interest in submachine gun is a relatively simple one. The upper echelons of the Japanese army did not appreciate the need for one. The doctrine of machine guns supported by infantry armed with bolt action rifles dominated Japanese tactical thinking. Another contributing factor was that until 1940 Japan had not fought in Jungles. It is a common misconception that the Japanese entered the war as experts in jungle warfare, and while they were hardy soldiers able to subsist on relatively nothing, this was not the case. There are no jungles on the Japanese mainland and the campaigns in which the Japanese had been involved in during the 1930s had all taken place far away from the jungles of the Pacific.

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Apart from the barrel, machining a SMG receiver is a costly and time consuming operation that stamping à la Sten / MP38 - 40 / M3 / PPSh greatly solved.
Also on the M1928 Thompson, the actuator and Cutts compensator are very complex and expensive parts.

Frog said:

Apart from the barrel, machining a SMG receiver is a costly and time consuming operation that stamping à la Sten / MP38 - 40 / M3 / PPSh greatly solved.

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Match the stamped receiver with the second-hand barrels and you're ready? Not that I'm inventing something new, though.

But the soviets were not using 2nd hand barrels?
They may have been taking a brand new barrel that had not been turned down on the outside and cutting them in 1/2 to make two short barrels, which were both turned down on the outside to the needed/desired shape.
A lot depends on the barrel making equipment that you have.
MK II Sten gun barrel.

You don't have the diameter needed to make the SMG barrel out of the front part/s of an existing rifle barrel. If your rifle barrels were that fat the guns would weigh 1-2kg more than they did. Only the rear part near the chamber (where the rear sight is on many bolt actions) is fat enough to get a one piece barrel for the SMG.
Again you can machine a sleeve to fit over the skinny barrel part but at what cost?

For cut rifled barrels they start with large diameter rods, drill them and rifle them and THEN cut the exterior dimensions. They often leave a large lump on both ends to center it in the lathe and cut to length.
If you try to make the barrel too skinny to begin with they tend to warp/bend while doing the drilling/cutting.
Trying to deal with skinny rifle parts that were cut off existing rifles would be a real pain in the ass.
You are trying make SMG by the 10s of thousands (or more?) not a few shop projects.
Tool up, make the guns and don't dink around trying to same a few yen while increasing actual labor costs.
If your machinery allows for it you can make short barrels if you want.
Colt Firearms actually for many years could NOT make a barrel longer than 8in. Their rifling machine only had an 8in stroke, if they sold a gun with a longer barrel they purchased the barrels from an outside company. Their machine would rifle either 6 or 8 barrels at a time, side by side.
Just because the Soviets did something, in one of their factories, for a short period of time, does not mean it was actually a good way to do it.

Shortround6 said:

You are trying make SMG by the 10s of thousands (or more?) not a few shop projects.
Tool up, make the guns and don't dink around trying to same a few yen while increasing actual labor costs.

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The 'tool up' tidbit kinda assumes that it was feasible on a whim and on a short notice.
Making several thousands of SMGs while using old, existing barrels, can save a lot of effort, coin, and, most important, time.

Shortround6 said:

Just because the Soviets did something, in one of their factories, for a short period of time, does not mean it was actually a good way to do it.

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It was an expedient, that helped them bridge the gap until the 'proper' SMG mass production was on the way.

peceed said:

The biggest ww2 "missed opportunity" for "heavy smg"/"proto-assault riffle" is Thompson 0.30 Carabine.

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That maybe shows a bit the limits of using more powerful rounds in a blowback gun. 12 lbs, yikes! (Not that the Thompson was particularly light even in pistol calibre.)

There seems to be a continuing misconception about choosing a calibre to match one's rifle calibre.

It is not to use up worn existing barrels. If your rifles have worn barrels you rebarrel them and reissue them. Machine carbines need good barrels just as much as do rifles. What you are doing is making more barrels on the existing barrel blank machinery and then cutting them to rifle or machine carbine length and exterior and chamber machining to suit the weapons in question.
If your barrel blank making can produce enough to cater for both weapons well and good. If not then you simply make more copies of the machinery and a new factory or extra building. In the case of the Soviets this made mass production a quick process to introduce and maintain.
However, in almost all other cases calibre was driven by existing pistol ammunition (and this reinforced the Soviet decision too) rather than barrel production. Using rifle barrel production as the reason presumes an existing surplus or ability to create a surplus over the needs of rifle production. Allied would be the industrial capacity to make new copies of the barrel blank making machinery.
One might note the problems the Italians might have with their weird 6.5 barrel twists but their soldered barrel liner system could be useful to somebody perhaps? Their 7.35 barrels used a simple twist IIRC but that ammunition was suited more to an intermediate semi automatic rifle and far too much for a machine carbine. But I digress.

In short, one cannot use old rifle barrels. One uses new barrel blanks and cut to the length for either the rifle or the machine carbine.

yulzari said:

In short, one cannot use old rifle barrels.

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There's the possibility to bore out worn barrels to some bigger size. Which might a (small, admittedly) reason to choose your SMG caliber larger than your rifle caliber.

That being said, I don't know if such boring out is suitable to high volume manufacturing, or would it be better to just melt down the old barrels and start from scratch?

tomo pauk said:

The 'tool up' tidbit kinda assumes that it was feasible on a whim and on a short notice.
Making several thousands of SMGs while using old, existing barrels, can save a lot of effort, coin, and, most important, time.

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But we're not looking at 'several thousands', are we? The Japanese would have needed a million, if not more, SMG's. They produced about 3 million of the 6.5mm Type 38 rifle, and were in the process of replacing these with the 7.7mm Type 99 rifle, of which they produced another 3 million. So I guess due to the ongoing conversion project they had plenty of 6.5mm barrels to use.

Of course, not a given this would have saved time vs just melting down the old barrels and manufacturing new SMG barrels in whatever caliber they had decided upon.

z42 said:

But we're not looking at 'several thousands', are we? The Japanese would have needed a million, if not more, SMG's. They produced about 3 million of the 6.5mm Type 38 rifle, and were in the process of replacing these with the 7.7mm Type 99 rifle, of which they produced another 3 million. So I guess due to the ongoing conversion project they had plenty of 6.5mm barrels to use.

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One (ad-hoc production in thousands) does not exclude the other (making a major effort to tool up in order to make millions, or at least hundreds of thousands of SMGs).

tomo pauk said:

One (ad-hoc production in thousands) does not exclude the other (making a major effort to tool up in order to make millions, or at least hundreds of thousands of SMGs).

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Indeed it doesn't, but with just a few thousands the impact on the war is going to be insignificant. So why bother, if you can't scale up the process?

z42 said:

Indeed it doesn't, but with just a few thousands the impact on the war is going to be insignificant. So why bother, if you can't scale up the process?

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In order to have thousands of SMGs in service today, while waiting for many more thousands from the proper production lines in the next year and on.

re machining of SMG barrels

Unless there is something weird about the rifle barrels, it should be significantly easier to bore out worn rifle barrels to a larger caliber - particularly if they are sectioned to shortened lengths first. Since the barrel is already bored all the way through, the original diameter can be used as a pilot hole for a push-through type drill and/or reamer. If the SMG caliber is only a few thousandths of an inch larger in diameter (say no more than .010"-.015") then it might be possible to finish/resize the bore in one operation with one pass. If the SMG barrels are short enough (eg 6"-10"), it might be possible to use non-specialized machines (eg regular lathes) and tooling for the drilling/reaming.

Rifling would be about the same as for a new barrel of a similar length - assuming approximately the same depths, twists, and tolerances.

What external machining would be required is a different question, and depends a lot on the design of the original rifle barrel (eg bulges and tapers that might need additional machining to deal with) and the SMG barrel assembly/retention design. As long as there is no problem holding the section of rifle barrel while machining, the external machining should take no more time than the that required for a new production barrel, and could take significantly less time.

The final machining of the chamber would be about the same, depending on the size and shape of the case.

SaparotRob said:

It would've been the coolest looking one.

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I like the look of this .22 smg and its circular magazine for max rounds.


View: https://m.youtube.com/watch?v=si7Q6BCPBYg

tomo pauk said:

More rounds can be carried for the same weight, so this is at least one benefit.

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That mag looks right off a slide projector. "It's not called the wheel. It's call the carousel." Don Draper.