The M1911A1 is a single-action, semi-automatic, magazine-fed, recoil-operated handgun chambered for the .45 ACP cartridge, which served as the standard-issue side arm for the United States armed forces from 1911 to 1985. It was widely used in World War I, World War II, the Korean War, and the Vietnam War. The M1911 is still carried by some U.S. forces. Its formal designation as of 1940 was Automatic Pistol, Caliber .45, M1911 for the original Model of 1911 or Automatic Pistol, Caliber .45, M1911A1 for the M1911A1, adopted in 1924. The designation changed to Pistol, Caliber .45, Automatic, M1911A1 in the Vietnam era. In total, the United States procured around 2.7 million M1911 and M1911A1 pistols in military contracts during its service life. The M1911 was replaced by the M9 pistol as the standard U.S. sidearm in the early 1990s, but due to its popularity among users, it has not been completely phased out. Modern M1911 variants are still in use by some units within the U.S. Navy and U.S. Marine Corps.