The M1A1 Thompson was a submachine gun developed in the United States. It was a common weapon used by the US Army and US Marine Corps. It was also popular with the British Army, British Marines and the SAS.
The M1A1 Thompson was similar in design to its predecessor, the M1928 Thompson, but was mostly revised for military use, and had a flat rifle style fore grip and removed muzzle brake. Another notable change was the use of only a 20 or 30 round box magazine, where as the M1928 could be equipped with drum magazines as well as box magazines. The M1A1 Thompson also had a slightly modified firing system, using straight blowback instead of delayed blowback. The M1A1 was also smaller, making it more compact and controlable, it measured 813 mm in complete length (32 inches), and the barrel measured 266 mm in length, and the weapon weighed slightly less than the M1928 when unloaded at 4,82 kilograms. Other than these differences, the M1A1 Thompson shared the same general characteristics as the M1928 Thompson, such as the 11.43 mm (.45 ACP) caliber and the 55 yard range. The M1A1 Thompson could fire up to 700 rounds per minute.
The M1A1 Thompson was designed by Savage Arms Corporation to replace the early M1928 Thompson. While the older weapon was not replaced entirely, the M1A1 was adopted by the US Marine Corps as their primary submachine gun the year of its introduction, 1942. The Army was not far behind, and chose the M1A1 Thompson as a general purpose infantry submachine gun to be used alongside other submachine guns of the day, such as the M3 Grease Gun and Reising Models 50 and 55. M1A1 Thompsons were available in large enough numbers to one the most widely used submachine guns during Operation Torch, the re-conquest of French North Africa. By 1944, the M1A1 Thompson had fully replaced the older M1928 Thompson except in the British and Canadian armies. The M1A1 was also in use by France's French Foreign Legion and army. The M1A1 Thompson remained in military service throughout World War II, and continued to be used by armies around the world until the 1970s.
https://www.youtube.com/watch?v=lW-IjtiVthc Thompson SMG in 30 Carbine (Forgotten Weapons)