I lifted this from wiki: The M1A1, standardized in October 1942 as the United States Submachine Gun, Cal. .45, M1A1, could be produced in half the time of the M1928A1, and at a much lower cost. The main difference between the M1 and M1A1 was the bolt. The M1 bolt had a floating firing pin and hammer, the bolt of the M1A1 had the firing pin machined to the face of the bolt, eliminating unnecessary parts. The reinforced stock and protective sight wings were standard. The 30-round magazine became more common. In 1939, Thompsons cost the government $209 apiece. By the spring of 1942, cost reduction design changes had brought this down to $70. In February 1944, the M1A1 reached a low price of $45 each, including accessories and spare parts, although the difference in price between the M1 and M1A1 was only $0.06. By the end of the war, the M1A1 was replaced with the even lower-cost M3 (commonly called the "Grease Gun"). The good ole Tommy Gun was popular with the boys; The police like them. The bad guys liked them too. They called them "choppers" and carried them around in violin cases. German paras liked them. The Tommies even liked the Tommy Gun! The Aussies at Tobruk seemed to like them as well. Even the Marines liked them, but I've read that US forces in the more equatorial areas of the Pacific the felt that the Thompson was not as effective in dense undergrowth. Seems that the low velocity .45 round didn't cut through the jungle underbrush very well. The BAR was far more suited for "Jungle Cleaning" and several were issued to each squad later in the war. Here the Devil Dogs are engaging the Nips on Okinawa. Not much jungle there though, so the Tommy Guns were well liked again.