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, and uses a large pistol primer. It has a case capacity of 25 grains. The bullet is .452 inches in diameter, and range in weight from 155 grain to 230 grain, though 250 grain is not unheard of in some lead cast loads. .45 ACP is fired optimally through a barrel with a 1/16 twist, and fires with a maximum pressure of 21,000 psi, producing a muzzle velocity of 825-850 feet per second.