The .45 Automatic and the .45 Colt are common names for two different handgun bullets that share the same caliber. When tracking the lineage of handgun development, the .45 ACP (Automatic Colt Pistol) could easily be considered a direct descendant of the .45 Colt, also known as the .45 Long Colt. Both are powerful cartridges for two distinct styles of handguns: the .45 ACP for auto-loading pistols and .45 Long Colt for revolvers.
The .45 Long Colt cartridge was first introduced after the Civil War and designed to be used in revolvers. The cartridge is commonly known as a .45 Long Colt to differentiate it from the shorter .45 ACP cartridge. The cartridge was widely used in single-action, big bore revolvers which are generally associated with the cowboys of the old west. The .45 Long Colt is the still a very popular cartridge and a staple of cowboy-style shooting.
The .45 ACP cartridge was developed by John Browning in the early 20th century as he was developing one of the first auto-loading pistols for the U.S. military. His creation, the Model 1911 pistol, and the cartridge he designed for that weapon, were adopted by the U.S. Army in 1911 and stayed in service until it was replaced in the early 1980s by the Beretta M9 9 mm pistol. TIn the 21st century, pistols chambered for .45 ACP are still extremely popular among law enforcement and civilian shooters.
The shell casing of the .45 Long Colt is cylindrical with straight sides. The cartridge is made to be placed into a revolver's cylinder and manually ejected. The cartridge has an overall length of about 1.6 inches (depending on manufacturer). The average muzzle velocity, for most commercially available cartridges, is 960 fps (feet per second).
The shell casing of the .45 ACP is angled and rimmed at the end to operate in a semi-automatic pistol. Unlike a revolver cartridge, which uses the shell’s rims to hold it in the cylinder, the rims of ammunition used in semi-automatic pistols (all of them, not only the .45 ACP) are designed to feed into the magazine and to guide the bullet into the pistol’s chamber. The cartridge has an overall length of approximately 1.25 inches and an average muzzle velocity of 830 fps.