The Colt M1911 (also known as 1911, Colt 1911 or Colt Government in the case of Colt-produced models) is a single-action, recoil-operated, semi-automatic pistol chambered for the .45 ACP cartridge.[9] 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.[9]

Designed by John Browning, the M1911 is the best-known of his designs to use the short recoil principle in its basic design. The pistol was widely copied, and this operating system rose to become the preeminent type of the 20th century and of nearly all modern centerfire pistols. It is popular with civilian shooters in competitive events such as the International Defensive Pistol Association and International Practical Shooting Confederation.[15]

The U.S. military procured around 2.7 million M1911 and M1911A1 pistols during its service life. The pistol served as the standard-issue sidearm for the United States Armed Forces from 1911 to 1985.[16] It was widely used in World War I,[17] World War II, the Korean War, and the Vietnam War. The M1911A1 was replaced by the adoption of the 9mm Beretta M9 pistol as the standard U.S. military sidearm in 1985. However, the U.S. Army did not officially replace the M1911A1 with the Beretta M9 until October 1986; production and procurement delays kept the 1911A1 in service with some units past 1989. The 1911A1 has never been completely phased out. Modernized derivative variants of the M1911 are still in use by some units of the U.S. Army Special Forces, U.S. Marine Corps and the U.S. Navy. The M1911 pistol originated in the late 1890s as the result of a search for a suitable self-loading (or semi-automatic) pistol to replace the variety of revolvers in service at the time.[18]