The original M1928s were M1921s with weight added to the actuator, which slowed down the cyclic rate of fire, a U.S. Navy requirement. With the start of World War II, major contracts from Britain and France saved the manufacturer from bankruptcy. This model was standardized as the M1928.
M1928A1
The M1928A1 variant entered mass production before the attack on Pearl Harbor, as on-hand stocks ran out. Changes included a horizontal forend, in place of the distinctive vertical foregrip ("pistol grip"), and a provision for a military sling. Despite new U.S. contracts for Lend-Lease shipments abroad to China, France, and the United Kingdom, as well as the needs of American armed forces, only two factories supplied M1928A1 Thompsons during the early years of World War II. The weapon was mostly used in the U.S. military by the Marine units in the Pacific Theater. Though it could use both the 50-round drum and the 20- or 30-round magazines, active service showed the drums were more prone to jamming and extremely heavy and bulky, especially on long patrols. 562,511 were made.
In addition, the Soviet Union received M1928A1s, included as standard equipment with the M3 light tanks obtained through Lend-Lease. The weapons were never issued to the Red Army, however, because of a lack of .45 ACP ammunition on the Eastern Front, and were simply put in storage. As of September 2006, limited numbers of these weapons have been re-imported from Russia to the United States as disassembled "spare parts kits", the entire weapon less the receiver (as required by Federal law).
M1
The M1, formally adopted as the United States Submachine Gun, Cal. .45, M1, was a result of further simplification. The bolt was modified and rate of fire was also reduced to approximately 600-700 rpm. The M1 utilized a simple blowback operation, the charging handle was moved to the side, and the flip-up adjustable rear sight replaced with a fixed aperture (peep sight). The slots adjoining the magazine well allowing use of the drum magazine were removed, as were the Cutts compensator, the barrel cooling flanges, and the Blish lock.
The less expensive and more-easily manufactured "stick" magazines were used exclusively in this version, with a new 30-round version joining the familiar 20-round type.
The M1 also has a permanently attached buttstock, and was first issued in 1942.
M1A1
Both sides of the Thompson M1A1
The multi-piece firing pin of the M1 was supplanted by a simplified firing pin machined into the face of the bolt. The 30-round magazine was very common. Reinforcement wings were added to the fixed rear sight.
The M1A1, formally adopted 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. In 1939, Thompsons cost the government $209 apiece. By the spring of 1942, cost reduction design changes had brought this down to US$70. In February 1944, the M1A1 reached a low price of US$45 each, including accessories and spare parts. By the end of 1944, the M1A1 was replaced with the even lower-cost M3 (commonly called "Grease Gun").
