How a M3 works

The M3 was an American .45-caliber submachine gun adopted for U.S. Army service on 12 December 1942, as the United States Submachine Gun, Cal. .45, M3.[6] The M3 was chambered for the same .45 round fired by the Thompson submachine gun, but was cheaper to produce, and lighter, although, contrary to popular belief, it was less accurate. This myth stems from a US Army training film portraying the M3 as more accurate than its counterparts.[7] The M3 was commonly referred to as the "Grease Gun" or simply "the Greaser," owing to its visual similarity to the mechanic's tool.

Intended as a replacement for the .45-caliber Thompson series of submachine guns, the M3 began to replace the Thompson in front-line service in mid-1944. Due to delays caused by production issues and approved specification changes, the M3 saw limited combat use in World War II and the M3A1 none. The M3A1 was used in the Korean War and later conflicts.