Too many cartridges wound up with similar names despite being totally different from one another. We often have shooters ask us if they can fire 38 Super in their 38 Special revolvers. It’s perfectly fair of them to wonder if they could – “super” and “special” are similar concepts, after all, and it doesn’t help when they’re preceded by the same number.

On the flip side, too many cartridges also have multiple, dissimilar sounding names. We get plenty of people asking if they can fire 9mm in a Glock 17 with “9×19” engraved on its barrel. Perhaps no two rounds’ names lead to more confusion than the 45 Long Colt vs. 45 Automatic Colt Pistol (ACP). We dread getting the phone call:

“Why won’t this ammo I ordered from you fit in my 1911?” We know the answer before they say anything further: “You got the wrong ammo!”

The Difference Between 45 Long Colt vs. 45 ACP

45 Long Colt

45 LC ammo loaded in a single shot revolver The 45 Long Colt was developed in 1872 in partnership by Colt and the Union Metallic Cartridge Company, which is now a subsidiary of Remington. Originally a black powder round by necessity, the U.S. Army adopted 45 Long Colt in 1873. It served as the official handgun round for their Colt Single Action Army revolver for 14 years.

45 ACP

45 ACP Glock with ammunition at the shooting range John Browning developed the 45 ACP in 1904. At the time the Army wished to replace their revolvers with semi-automatics, but naturally without sacrificing accuracy or power in the process. As the Thompson-LaGarde Tests of 1904 had determined .45 caliber to be the most effective pistol cartridge, Browning kept that bullet.

Browning’s 1911 pistol remained the main sidearm of the U.S. military until its replacement by the Beretta M9 in 1985 and then the 9mm SIG Sauer P320 in 2017. The primary difference between the 45 Long Colt and 45 ACP is readily apparent: One is designed for a revolver, while the other is designed for a semi-auto. The 45 Long Colt’s shell casing thus features a rim that enables it to sit securely in a cylinder.

While plenty of semi-autos can handle rimmed cartridges – the 22 LR being the most popular that comes to mind – rims are generally considered a detriment to smooth feeding and extraction. While 45 ACP ammo technically does have a rim, its rim diameter is eight times smaller than that of the 45 Long Colt’s when you compare the rim’s protrusion to the base diameter of the cartridge.

As a dedicated semi-auto cartridge, the 45 ACP features an extractor groove that provides a surface for a handgun’s extractor to purchase against during extraction. You will not find such a groove on a revolver cartridge. If you would rather differentiate the two cartridges by their lengths alone, the 45 Long Colt is 0.325” longer.

The Similarities Between 45 Long Colt and 45 ACP

45 LC and 45 ACP ammo side by side

A close-up shot of 45 Long Colt ammo on the left and 45 ACP ammo on the right.

John Browning didn’t set out to create an entirely different cartridge than that which the U.S. Army had become accustomed to. He actually developed the 45 ACP to match the ballistics of the 45 Long Colt fairly closely! It’s not an apples to apples comparison, though. A typical 45 Long Colt cartridge has a 250 grain bullet and 860 fps muzzle velocity, giving it 411 ft lbs muzzle energy.

In comparison, a standard 45 ACP cartridge has a 230 grain bullet and 850 fps muzzle velocity, giving it 369 ft lbs muzzle energy. The 45 Long Colt is effectively a more powerful cartridge, but with a 1911 you get seven shots in the magazine instead of six in the cylinder. It’s a lot easier to reload a semi-auto than a revolver when you’re in a hurry, too, which is why they’re generally more favored. The 45 Long Colt has a .452” bullet.

The 45 ACP, on the other hand, has a .451” bullet. If you consider one one-thousandth of an inch a great length, then you may argue that the two cartridges’ bullets are in fact very different from one another. It’s going to be a tough sell, though.

The Takeaway

45 LC revolver with federal ammo laid out at the range It’s dangerous to mistake some rounds for others when they share identical dimensions yet create very different chamber pressures. That’s why you should take every precaution against running 5.56×45 through a rifle chambered only for 223 Rem, and why you also don’t want to run 9mm +P+ through an original Luger pistol.

Fortunately, you’re not likely to mistake a rimmed 45 Long Colt for a rimless 45 ACP. They simply look too dissimilar to permit such a mix-up. Simply avoid buying 45 ACP for your Single Action Army, or 45 Long Colt for your semi-auto, and you’ll be safe. The only difficult task remaining will be actually hitting your target!