During the height of the ammo shortage during the Obama administration, my local big-box gun store had a policy that customers could only buy one box of each cartridge at a time. When I carried a box of .45 Colt and a box of .45 ACP to the counter, the young lady who was running the register, who obviously knew nothing about firearms, refused to let me buy both because “they’re clearly the same—they both say .45.” It was frustrating, but she wasn’t interested in being educated, and I walked out with a single box of .45 ACP.
What that young woman didn’t know is that these are two dramatically different cartridges, and they cannot be interchanged.
The .45 Colt (sometimes colloquially called Long Colt) was designed in the 1870s for revolvers. It was and still is very much associated with cowboys and the Old West. The .45 ACP (Automatic Colt Pistol), on the other hand, was designed for semi-automatic handguns in the early 1900s by the legendary John Moses Browning. It became wildly popular after it was picked up by the military, particularly for use with the 1911 pistol. It’s still a very popular cartridge today.
Though both are .45-caliber cartridges, the cases are different, and this means the two cannot be interchanged or shot out of the same gun. The .45 ACP has a shorter case and operates at a higher pressure. The .45 Colt, since it is a revolver cartridge, doesn’t need to worry about cycling a semi-automatic action. It’s a capable hunting cartridge when loaded properly, and its versatility makes it popular among handloaders. It can generally handle heavier bullets than the .45 ACP, too.
It's important to note that if you own a vintage revolver, you should be careful shooting modern .45 Colt cartridges out of it. The cartridge was originally designed for blackpowder, but today the ammo is loaded with modern smokeless powders, which generate considerably higher pressures. That’s fine in modern guns, but vintage firearms were not designed for such high pressures, and shooting modern .45 Colt loads out of a vintage revolver could damage the gun and injure the shooter.
Bottom line: These two cartridges are quite different, despite being the same caliber. The biggest practical difference is the guns they’re fired out of: The .45 Colt is used in revolvers, while the .45 ACP is used in semi-automatic handguns. The .45 Colt is rimmed and considerably longer than the rimless .45 ACP, so it’s quite easy to tell the difference when looking at the two cartridges side by side.
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