When World War II started many M1928 Thompsons were modified as the M1928A1 variant, but the production costs were high and production slow.
A new version was developed as the M1A1 with the front grip changed, the cocking lever moved from the top to the side and the bolt simplified. It only took box magazines as the 50 and 100 round drum magazines were considered heavy, noisy, prone to jamming and slow to load. The 30 round stick magazine (shown) was most common.
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