In order to simplify the M1928 Thompson, development on the M1 began in 1942. The M1’s were issued with a 30-round box magazine and could use the earlier 20-round box. The M1 also had a barrel without cooling fins, a simplified rear sight, and had a straight blowback action. The majority of the Thompsons issued for WWII were the simplified M1 and M1A1 variants.