Ah, yes, the M1A1 Thompson in US service. Yes, I do remember that it was still being used by the United States in TL-191, as well as the Springfield 1903. Seems like a few recognizable US weapons are still in US service in TL-191. A critical weapon missing is the M1 Garand it seems.
Hm. Have no idea what caliber the CSA would be using in their submachine-guns. Since the .45 was in service with the US, the CS would develop their own caliber. While I can't say the exact round, I can probably give a guess as to the design philosophy - as in "Hey y'all, what kind of bullet is powerful enough to stop a Yankee dead in his tracks?"
You know this already for sure - in our timeline the US military needed a handgun and a bullet that had more stopping power and, overall, a better reload rate based on their experiences in the Philippines - they needed "man-stopping" rounds and guns that could fire them in a more fictive way.
The Confederates in TL-191 by 1941 have the knowledge and experience of three major wars under their belt, with the Great War of 1914-1917 and the Mexican Civil War of the 1920s being their most relevant "testing grounds" for effective ammunition. My best guess is that the Confederates would need a round that is reliable enough in poor environmental conditions, can stop a man with enough force to put him down in close quarters, and can be reliably fired from a pistol and a submachine-gun. Furthermore, they would have to design guns that can be interchangeable with the same round to ease supply issues and logistics - the thinking being "hey, lets make some guns that can fire the same reliable round".
Maybe that's a good idea, maybe not. I just feel like the Confederates would be big on designing a pistol round that can put a person down and put them down hard, able to be used in their SMGs too. No, not the .45 ACP of course, but perhaps any kind of round that can do the job of stopping a Yankee dead.
Him, an SMG with a butt-stock huh?
Well
@cortz#9 here made a very handsome submachine-gun a while ago that I really like that has a wooden butt-stock. Its a fictional design of course, but hey.
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^^^ --- This is
@cortz#9 design. I like calling it the "Griswold".
Other than that I have no idea what the guards would be using as a submachine-gun with a wood-stock and a drum magazine...
... maybe a Suomi type submachine-gun?
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