So, the Reising SMG had two kinds of mags, one was a 20 round magazine and another was a 12 round magazine. The 12 round magazine had stamped in ridges (as opposed to the flat walled 20 round mags) which forced rounds in the magazine into a single stack.
On the top of the following image is an M50 with a 12 round magazine, and on the bottom is one with a 20 round mag.
In addition, some M50 Reisings had built in restrictors in their mag wells which limited the weapons to only accepting 12 round magazines. This is the model that seems to be in game.
Hehe 12 rounds. Even less people would use 
The Marines hated the thing IRL too
I actually like the m50. It is an wannabe thompson that i use in close quarters.
M50 Reising is one of the best SMGs of US togethet with M1921 and M1928 Drum
It’s not terrible in game, but irl it had some serious reliability issues. It was basically a civilian firearm push into military service, and it didn’t stand up so well to battlefield conditions.
