A real M1A1 Thompson only shot 20 or 30 rd magazines. The firing pin was cut into the bolt which is a 2 lb (a guess) bolt assembly (a big ass bolt) that flung back and forth. Basically a slam fire operation. It’s heavy for sure, but not in the 50 lb range.
Weight
10.8 lb (4.9 kg) empty (M1928A1)
10.6 lb (4.8 kg) empty (M1A1)
Length
33.5 in (850 mm) (M1928A1)
32 in (810 mm) (M1/M1A1)
Barrel length
10.5 in (270 mm)
12 in (300 mm) (with cutts compensator)