I know a better system, ill use the Thompson as an example.
You start out with the worst model available and work your way up to the best model and have to level it up to unlock historicaly accurate components.
M1A1 Thompson starts at 20 Rounds and can be upgraded to 30 Rounds (Compensators and drums not available for this model.
M1 Thompson starts at 20 Rounds and can be upgradd to 30 Rounds (Compensators and drums not available for this model)
M1928A1 Thompson starts at 20 rounds and can be upgraded to 30, 50, or 100 rounds. (M1928A1 also has different variants with a compensator and barrel fins)
M1921/28 Starts at 20 rounds and can be upgraded to 30, 50, or 100 rounds (model has non removeable compensator, accepts drums, and has non removeable barrel fins)
M1921 starts at 20 rounds and can be upgraded to 30, 50, or 100 rounds (model accepts drums and compensators, has a RPM of 1300)
I didn’t mention the BSA M1929 or M1928 USMC but they Should be considered as well.
Also realism will play a role in this, smaller magazines will reload faster and weigh less.
Larger magazines will slow down aiming speed and reload speed and make you move slower.
Also it may be unfair to run three squads with 100 round Thompson M1921AC that have a RPM of 1300 since the Thompson kills in 1 shot at close range so keep that in mind.
Also another example would be the Beretta SMGs, the magazine sizes go from 10, 20, 30, and 40 rounds in the Beretta M1, M38, M38/42, M1918/30, and FNAB-43.
The Type 2A SMG uses the same magazine as the Type 1 so a 50 rounder should be available.