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The .45 ACP (Automatic Colt Pistol) or .45 Auto (11.43×23mm) is a rimless straight-walled handgun cartridge designed by John Moses Browning in 1904, for use in his prototype Colt semi-automatic pistol. After successful military trials, it was adopted as the standard chambering for Colt's M1911 pistol. The round was developed due to a lack of stopping power experienced in the Moro Rebellion in places like Sulu. The issued ammunition, .38 Long Colt had proved inadequate, motivating the search for a better cartridge. This experience and the Thompson–LaGarde Tests of 1904 led the Army and the Cavalry to decide that a minimum of .45 caliber was required in a new handgun.
The standard issue military .45 ACP round uses a 230-grain (14.9 g) round nose projectile that travels at approximately 830 feet per second (250 m/s) when fired from a government-issue M1911A1 pistol. It operates at a relatively low maximum chamber pressure rating of 21,000 psi (140 MPa), compared to 35,000 psi (240 MPa) for both 9mm Parabellum and .40 S&W, which due to a low bolt thrust helps extend the service lives of weapons. Since standard-pressure .45 ACP rounds are subsonic when fired from handguns and submachine guns, it is a useful caliber for suppressed weapons to eliminate the sonic boom.
Today, most NATO militaries use sidearms chambered for the 9×19mm Parabellum cartridge, but the effectiveness of the .45 ACP cartridge has ensured its continued popularity with large-caliber sport shooters, especially in the United States. In 1985, the .45 ACP M1911A1 pistol was replaced by the Beretta M9 9 mm pistol as the main sidearm of the U.S. military, which in turn was replaced with the SIG Sauer P320 9 mm pistol in 2017, designated M17 for the full-size and M18 for the compact.