The M1911 (Colt 1911 or Colt Government) is a single-action, recoil-operated, semi-automatic pistol chambered for the .45 ACP cartridge. The pistol's formal U.S. military designation as of 1940 was Automatic Pistol, Caliber .45, M1911 for the original model adopted in March 1911, and Automatic Pistol, Caliber .45, M1911A1 for the improved M1911A1 model which entered service in 1926. The designation changed to Pistol, Caliber .45, Automatic, M1911A1 in the Vietnam War era.