A very prominent and sometimes 'fancy' weapon in the ww2, SMGs came in many layouts and in a number of cartridges. Some were better than others, some offering the 200m efficient range vs. others with 150 or even 100m (although the metrics of deciding the exact effective range seem lacking). Some started with drum magazines, by the end of ww2 most were using 'stick' magazines, again some mags being better than other.
So - let's 'make' as good SMG as possible. Either a mod of OTL weapon, or something that emerged after ww2. Cartridge choice - anything plausible for technology of ww2, no need to stick with what was available in a specific country back then. Size, weight, RoF are important, so is reliability and suitability for mass production. It will be good if effective range should come close to 200m, although 150m seems like a good average so we don't over-stretch the barrel. A 'simple blowback' operation, as it was the case with the SMGs of the day that mattered (keeps the wepon simpe and cheap; does not work well with cartridges like it was the .30 Carbine or 'stronger').
So - let's 'make' as good SMG as possible. Either a mod of OTL weapon, or something that emerged after ww2. Cartridge choice - anything plausible for technology of ww2, no need to stick with what was available in a specific country back then. Size, weight, RoF are important, so is reliability and suitability for mass production. It will be good if effective range should come close to 200m, although 150m seems like a good average so we don't over-stretch the barrel. A 'simple blowback' operation, as it was the case with the SMGs of the day that mattered (keeps the wepon simpe and cheap; does not work well with cartridges like it was the .30 Carbine or 'stronger').