Chamber pressure in the context of ".45 ACP"

The .45 ACP (Automatic Colt Pistol), also known as .45 Auto, .45 Automatic, or 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 cartridge.

The standard-issue military .45 ACP cartridge uses a 230 gr (15 g; 0.53 oz) round-nose bullet at approximately 830 ft/s (250 m/s) 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 as it lacks the sonic boom inherent to supersonic rounds.