The M3 is an American .45 caliber submachine gun that uses the .45 ACP (11.43x23mm ACP) pistol cartridge. It entered US Army service on December 12, 1942 as the United States Submachine Gun, Cal. .45, M3 and began to replace the .45 caliber Thompson series submachine guns: the M1928A1, M1 and M1A1 that were slowly being withdrawn from use. The weapon’s designer was G. Hyde, while F. Sampson – GMC’s Inland Division chief engineer – was responsible for preparing and organizing production. Even at the development stage, the weapon’s design was focused on simplified production, ease of use and the ability to convert the weapon to the 9x19mm Parabellum pistol cartridge. The weapon is commonly referred to as the "grease gun", owing to its similarity in appearance to the common mechanic's tool.