Originally supposed to be an automatic rifle to replace the M1903 springfield, the Thompson ended up becoming one of the most iconic submachine guns in history due to it's widespread use by gang members and policemen during the 20's. In 1928,the first real military version began production, the M1928.
This was a very expensive weapon, and it was thus simplified in the M1928A1. This one was further simplified in the M1,and later on the M1A1,with the cost of each gun having gone down from 209$ to 45$. The M1A1 was a gun designed to be relatively easy to use by soldiers, it was simple to regularly operate (didn't even need to be cocked after loading from an empty mag). It didn't have any complicated controls, it was easy to field strip (relatively), and it used reliable 30 round mags.
This was the main submachine gun of the USA army in the late war, and it was one of the weapons most used by American paratroopers all throught the war
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