The M2 4.2-inch mortar was a U.S. rifled 4.2-inch (107 mm) mortar used during the Second World War, the Korean War, and the Vietnam War. It entered service in 1943. It was nicknamed the "Goon Gun" (from its large bullet-shaped shells, monopod, and rifled bore – like a rifle for shooting Goons) or the "Four-Deuce" (from its bore size in inches). In 1951 it began to be phased out in favor of the M30 mortar of the same caliber.