The M3 Grease Gun: America’s Simplest Submachine Gun The M3 “Grease Gun” was a .45-caliber submachine gun developed by the United States during World War II as a cheaper and simpler replacement for the Thompson submachine gun. Introduced in 1942, it was officially designated the Submachine Gun, Caliber .45, M3. Its nickname came from its resemblance to a mechanic’s grease gun. The U.S. Army wanted a weapon that could be produced quickly, used minimal materials, and still provide reliable close-r

M3/M3A1 Grease Gun