The .45 ACP (Automatic Colt Pistol) and .45 Long Colt (sometimes referred to as .45 Colt or .45 LC) are two distinct cartridges with different histories, dimensions, and areas of use. Here are the primary differences between them:

1. Origin and History:

   • .45 ACP: Developed by John Browning in 1904, the .45 ACP is a centerfire cartridge designed for semi-automatic pistols. It was created to meet the needs of the United States military, and it served as the standard caliber for the Colt M1911 pistol.
   • .45 Long Colt: Introduced in 1872, the .45 Long Colt is a rimmed cartridge originally designed for use in revolvers. It was developed as a joint effort between Colt's Manufacturing Company and the Union Metallic Cartridge Company (UMC).

2. Cartridge Design:

   • .45 ACP: The .45 ACP is a straight-walled, rimless cartridge that headspaces on the case mouth. It has a larger case diameter than the .45 Long Colt.
   • .45 Long Colt: The .45 Long Colt is a straight-walled, rimmed cartridge that headspaces on the rim. It has a longer case and a larger rim diameter than the .45 ACP.

3. Dimensions and Ballistics:

   • .45 ACP: The bullet diameter of the .45 ACP is typically .451-.452 inches. It has a shorter overall length and operates at relatively high pressures. Standard loads typically use jacketed or lead bullets with a range of bullet weights.
   • .45 Long Colt: The bullet diameter of the .45 Long Colt is typically .451-.454 inches. It has a longer overall length, and the case can accommodate heavier powder charges. The .45 Long Colt can be loaded to higher velocities and is known for its versatility in bullet weights, ranging from light to heavy.

4. Applications and Use:

   • .45 ACP: The .45 ACP is widely used in semi-automatic pistols for self-defense, law enforcement, and competition shooting. It has a reputation for good stopping power and is commonly chambered in various handgun models.
   • .45 Long Colt: The .45 Long Colt is primarily associated with revolvers, such as the Colt Single Action Army (often referred to as "Colt Peacemaker"). It is used for cowboy action shooting, hunting, and recreational shooting. The .45 Long Colt offers a wide range of bullet options suitable for different purposes.

It's important to note that the .45 ACP and .45 Long Colt are not interchangeable due to differences in case dimensions, rim types, and chamber specifications. Always use the appropriate ammunition specified for your firearm to ensure safe and proper operation.