The M3 is 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.

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Cheaper to produce, lighter, and using a .45 round, the M3 was intended to replace the more expensive and heavier Thompson. On average in 1943, the M3 cost $15 per unit to produce.

The M3 began to enter front line service in mid 1944, but Due to delays caused by production issues and approved specification changes, the M3 saw limited combat use in World War II. A total of 622,163 M3/M3A1 submachine guns were assembled by the end of World War II

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The M3 was commonly referred to as the "Grease Gun" or simply "the Greaser," owing to its visual similarity to the mechanic's tool.

The M3 and M3A1 were largely withdrawn from U.S. front line service in 1959 and into the early 1960s, but continued to be used until the mid-1990s aboard armoured vehicles

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