Jim,
Weight of he M3 was 8.15 lbs empty vs 10.6 lbs for the Thompson. I fired a Thompson in the army as it was still in some inventories in 1965. Heavy? Only 1 lb more than my M14 empty. Loaded, the M14 weighed more.
The thing that impressed me, besides the strength and experience it took to fire it, was the accuracy on single shot fire. The M3 was long gone so I never got a chance to compare.
Dave
