Had a M1A1 while in Vietnam, traded for it from a ARVN sergeant. That one was made by Savage, WW2 vintage (also saw variations by Colt and Auto Ordnance). Relatively crude construction, compared to the 1921 and 1928 models. No compensator, stamped sheet metal rear sight, 20-round stick magazines only (would not accept the drums). Reasonably accurate to about 75 yards or so, not great at longer ranges even on semi-auto. Heavy to carry, but the weight provided pretty good control on full-auto.

Couple of years later I was on a city police department with 6 Model 1921's sitting in the armory. Apparently it had been quite some time since there was anyone who knew how to properly field-strip and clean them. I spent a few days doing that, then test-firing at the range, re-cleaning and putting them back in the vault. Shortly thereafter some genius arranged to trade them in on Colt AR-15's; IIRC the department got $300 each in trade allowance. We had a small mountain of surplus GI ammo including WW1 (1917 and 1918 headstamps) and WW2 (both brass and steel cased). I was able to liberate as much as I wanted for $0.05 per round, which I used for several years and continue to reload the brass cases now over 40 years later.

Shortly before I retired (1995) I saw Century International Arms flyers offering M1 and M1A1 Thompsons to law enforcement agencies only, priced at $250 each. Tempting, but no way to ever transfer for private ownership.