SMGs in ww2 with much smaller cartridges?

[z42]

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.

 

[Frog]

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.

 

[tomo pauk]

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.

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

 

[Shortround6]

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.