.45 Automatic Colt Pistol

The .45 Automatic Colt Pistol, or .45 ACP, is a pistol cartridge created by the famous ammunition designer John Browning in 1904. It is a straight and rimless cartridge that has a bullet diameter of 11.5 mm, base diameter of 12.1 mm, neck diameter of 12 mm, rim diameter of 12.2 mm, and case length that measures 22.8 mm. Its creation began when the US Cavalry needed a .45 caliber equivalent of Colt's .41 caliber pistol rounds. Samuel Colt and John Browning worked together in designing the requested cartridge, and later came up with the 1905 Colt Model gun and the .45 ACP.

Two ammunition experts vied for the manufacture of the .45 ACP for the U.S. Army, John Browning and Arthur Savage. Browning's design was submitted under Colt's Company, while Savage had a company of his own. In 1910, in a series of cartridge examinations, Browning's design qualified in all stages with no single error, while Savage's design committed 37 stoppages or parts failures.

The cartridge utilizes a 15 g bullet and has muzzle velocity of 260 meters per second. It has the similar firing capacity of a .45 S&W revolver cartridge, although its shortness makes it slightly less powerful than the S&W loads.