The .45 ACP (.45 Auto) is a large caliber rimless centerfire pistol cartridge. It has been in service since before World War I, and saw popularity when it was utilized in the Thompson SMG. Although it is not as long or as powerful as the .44 Magnum cartridge (due to the fact that magnum cartridges use more propellant and fire at higher pressures), it is slightly larger in diameter. The .45 ACP is one of the most popular pistol cartridges in production today, having had countless firearms chambered for it over its lifetime.
The .45 Automatic Colt Pistol (ACP) cartridge was developed for the Browning designed Colt Model 1905 automatic pistol, which was a development of the earlier Model 1903 pistol, in .32 caliber.
Since the Thompson-LaGuard Board of 1904 recommended the nothing smaller than .45 caliber be considered for the new service automatic, this cartridge was adopted as the new service pistol round.
Original military specifications for the Cartridge, Ball, Caliber .45, M1911 as adopted by the Ordnance Department were a 230gr FMJ roundnose lead-cored bullet, with jacket of copper, copper-washed mild steel, gilding metal, or cupronickel, loaded over a charge of 5.0gr Hercules Powder Company "Bullseye" propellant. This combination was specified to produce 825–850 feet per second from the muzzle of a 5" barreled M1911 service pistol. This duplicates the ballistics of the blackpowder .45 S&W Schofield cartridge that it replaced in service.
The .45 ACP is a rimless, straight-walled cartridge using a large pistol primer. It has a case capacity of 25 grain H2O. Jacketed bullets are .451" in diameter (.452" for cast bullets) with a weight range from 155 grain to 230 grain (although lead cast bullets are available up to 250 grain.) The most common rifling twist rate for .45 ACP is 1/16, the maximum pressure is 21,000 psi (SAAMI) for regular loads and 23,000 psi for +P loads (SAAMI.) Muzzle velocities range from approximately 800 to 1000 feet per second.