High Quality Content by WIKIPEDIA articles! 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. The M3 was a superior alternative to the Thompson submachine gun as it was cheaper to produce, lighter, more accurate, and was also chambered in .45 ACP. The M3 was commonly referred to as the "Grease Gun" or simply "the Greaser", owing to its visual similarity to the mechanic`s tool.