.45 ACP

The .45 ACP (Automatic Colt Pistol) or 45 Auto (11.43x23mm) 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 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 calibers 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 (145 MPa), compared to 35,000 psi (241 MPa) for both 9mm Parabellum and 40 S&W, which due to a low bolt thrust helps extend the 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.