45 auto

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 in Sulu experienced in the Moro Rebellion using the .38 Long Colt. This experience and the Thompson–LaGarde Tests of 1904 led the Army and the Cavalry to decide a minimum of .45 caliber was required in a new handgun.
The standard issue military .45 ACP round has a 230-grain bullet that travels at approximately 830 feet per second when fired from the 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 service life of weapons. Due to standard-pressure .45 ACP rounds being inherently 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 9mm pistol as the main sidearm of the U.S. military, which in turn was replaced with the SIG Sauer P320 9mm pistol in 2017, designated M17 for the full-size and M18 for the compact.

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