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Mention the words .45 or 1911, and most gun enthusiasts will know what you’re referring to and will most likely have some invaluable piece of information to share with you on the subject. Oddly enough, very few people know that the 1911’s can use the .45ACP and .45 Auto ammunition.

The .45ACP and .45 Auto ammunition are one and the same thing. Confusion also arises through the manufacturers using different naming and branding for the same ammunition. Colt originally named the bullet .45ACP, while SAAMI refers to the bullet as .45 Auto to avoid any copyright issues.

Ammunition packaging can be pretty confusing at times. Some call the .45 pistol ammunition .45 ACP, while others brand the ammo as .45 Auto. Is it the same thing, and is the ammo interchangeable in your gun? Read on to clear up the confusion.

Navigation

.45 ACP Versus .45 Auto – What’s The Difference?
What Is The Difference Between .45 Colt and .45 ACP?
The Differences Between The .45 GAP And The .45 ACP
The Difference Between .45 Colt, .45 Auto, and .45 GAP
Conclusion

.45 ACP Versus .45 Auto – What’s The Difference?

The .45ACP cartridge was released commercially in 1904 through the collaboration of the two legendary firearm and ammunition designers, Samuel Colt and John Moses Browning. Colt built the 1911 gun while Browning designed the ammunition. They did a great job, and I’m sure we can all agree.

The large bore, heavy-hitting self-defense cartridge was named the .45 Automatic Colt Pistol, abbreviated to .45ACP.

So now, to answer the question, the .45ACP and the .45 Auto are the same cartridges. The adopted naming came about when the Sporting Arms and Ammunition Manufacturers’ Institute was founded in 1926.

SAAMI made a conscious decision to avoid any legal issues when setting the standards for Ammunition specifications and named the .45ACP cartridge the .45 Auto in their reference material.

Ammunition manufacturers have over time adopted the name for use on their ammunition packaging, sometimes on using the .45 Auto naming and sometimes using both .45ACP and .45 Auto, or of course only .45ACP.

The various ammunition manufacturers went a step further and gave the cartridge their own names depending on where in the world the ammunition was manufactured. Below are a few more names given to the .45ACP cartridge. All are the same and safe to use in any.45ACP chambered guns.

11.43 x 23mm (Europe)
11mm 43 (France)
11.25mm (Argentina)

What Is The Difference Between .45 Colt and .45 ACP?

The .45 Colt cartridge was developed by a partnership between Colt and the Union Metallic Cartridge Company in 1872. The cartridge became successful when the Army adopted the Colt Single Action Revolver chambered for black powder .45 Long Colt cartridge in 1873.

Both the .45 Colt (Long Colt) and the .45ACP (Automatic Colt Pistol) use the same diameter bullet heads, namely, .451 inch (11.45mm) for jacketed bullets and .452 inch (11.48mm) for lead bullets.

The main difference between the cartridges is that the .45 Colt is made for use in a revolver. The cartridge is rimmed and longer than the .45ACP. The .45ACP is a rimless cartridge designed for use only in automatic pistols.

The .45 Colt and the .45ACP are two very different cartridges and are not interchangeable. The use of .45 in the cartridge’s name causes confusion and the assumption that the cartridges are the same, which of course, they are not.

The Differences Between The .45 GAP And The .45 ACP

The .45 GAP (Glock Auto Pistol) is a proprietary cartridge designed in 2002 by Ernest Durham, an engineer working for CCI / Speer. The brief from Glock was to develop a cartridge that would match the performance of the .45 ACP cartridge and allow for a more compact pistol design.

The 200gr .451 inch (.45) caliber round achieved 1,000ftps. The case length is .755 inches (19.2mm), with the overall length being 1.070 in (27.2mm).

The .45ACP case length is .898 in (22.8mm), firing a 200gr bullet at 900ftps.

Despite the shorter case length, the .45GAP achieves a slightly higher velocity than the .45ACP. The higher velocity is achieved by loading a higher velocity propellant into the .45GAP. The other interesting fact is that the .45GAP uses a small pistol primer which is good news for reloaders.

The two calibers are both .45 in diameter but do not compete directly with each other. The .45ACP has found its niche firing a 230gr bullet at 850ftps.

The preference of caliber choice for the average enthusiast lies more in the pistol offering than the actual bullets’ performance. The .45GAP does its best work with 200gr bullets or less. Glock was brilliant to introduce their cartridge for the Glock as they have a dedicated following.

Glock fans are more likely to choose the .45GAP offering, while 1911 followers are more likely to select the .45ACP.

The two cartridges are not interchangeable as the .45ACP is too long to fit into the chamber of a .45GAP. Likewise, the .45GAP is too short and operates at a higher pressure than the .45ACP should you be tempted to try anyway.

The Difference Between .45 Colt, .45 Auto, and .45 GAP

All three of the above cartridges are, of course, .45 caliber. The most obvious difference is that the .45 Colt cartridge is a rimmed cartridge made for use in revolvers. The .45 Auto (ACP) and the .45 GAP are designed for pistols only.

The .45 Colt is an excellent caliber. The cartridge’s versatility by being available in 200gr to 280gr bullets of various designs makes the .45Colt as popular today as it was in 1878. Using 250 to 255gr bullets, the velocity of between 850 ftps to 900 ftps will resolve pretty much any problems that the average civilian will encounter.

The .45Auto or ACP is the most popular caliber amongst the Colt and the GAP. The .45ACP is only outsold annually by the 9mm Luger. Not bad going for a cartridge that’s been around since 1904.

The .45ACP cut its teeth, so to speak, in numerous conflict situations.

For seventy years, the US Military used the .45ACP as their official sidearm caliber. This is a testament to the effectiveness and quality of the pistols firing the round. Amongst the notable wars where the .45ACP saw active duties were both world wars and Vietnam.

The 230grain bullet in the .45ACP caliber is the standard against which many other calibers are measured. The Colt 1911 pistol made the .45ACP (Auto) famous, or is it the other way round? Either way, they make a great team.

The .45 GAP (Glock Auto Pistol) cartridge is the new kid on the block. It was designed specifically for use in the excellent Glock Pistol. The cartridge has found a following among Glock pistol fans and .45 caliber shooters.

The .45GAP was designed and commissioned by Glock to take advantage of the large following of .45CAL shooters.

The .45GAP caters to the best of both worlds. Modern Glock pistol design offers a compact .45 caliber bullet traveling faster than the 230gr .45ACP offering.

Conclusion

The .45 ACP and .45 Auto cartridges are exactly the same cartridges. The .45 Automatic Colt Pistol cartridge has been in production since 1904.

SAAMI inadvertently created the confusion many decades ago when they decided to call the .45ACP cartridge the .45Auto to void any patent disputes arising from the publication of the cartridge dimensions as part of their work.