The Colt M1921 Thompson is a usa-made select-fire submachine gun chambered in .45 ACP, introduced in 1921. Primary use: Military. Type: Submachine Gun Action: Select-Fire Caliber: .45 ACP Capacity: 20 Year introduced: 1921, USA Weight: 88 oz Overall length: 24" Status: Discontinued Military use: Yes Intended use: Military