What Does ACP Stand For In Ammo

What Does ACP Stand For In Ammo?

ACP is a term that you’ve likely seen placed next to calibers (such as ‘.45 ACP’).So what does ‘ACP’ mean and why is it a big deal?

Those who are new to the world of guns and ammo may understandably want an answer to this question, and if you’re one of those people, you’re reading the right article.  

To answer this question, we need to take you back in time to the early 1900s. This was a time when a gentleman named John Moses Browning was making his mark on the firearms world.

John Browning and his ACPs

John Browning and his ACPs

The short answer to the question of what ACP stands for is ‘Automatic Colt Pistol.’

This does not mean, however, that rounds designated as ACP can only be chambered in automatic Colt pistols. Let’s explain. 

There are many calibers that are designated as ACP, including .25 ACP, .32 ACP, .38 ACP, .380 ACP, and .45 ACP. We’ll discuss each of these calibers later in this article.

In the early 1900s, famous and influential firearms inventor John Browning was working for Colt designing semi-automatic pistols. As part of his design for these pistols, he also designed the aforementioned calibers that would be fired in them.

These calibers were designated as ‘Automatic Colt Pistol’ or ‘ACP’ simply because they were designed, originally, for use in semi-automatic Colt pistols. However, other firearms manufacturers would produce weapons that could fire cartridges designated as ACP as well.

Here is an example: the .45 ACP cartridge was originally designed for the Colt M1911 pistol. But there are countless other non-Colt firearms that can chamber and fire .45 ACP ammunition as well, including the HK USP, SIG Sauer P220, and many others.

John Browning was the most legendary, prolific, and influential firearms inventor ever born. We can thank him for guns ranging from the Winchester 1886, 1892, 1894 and 1895 lever action rifles; the Auto 5 shotgun; the Colt M1911 and Browning Hi-Power pistols; the Browning Automatic Rifle (BAR); and the Browning M2 machine gun. Most of these guns are still in wide use today. 

Browning was also the first to design several semi-automatic and self-loading pistols. He got the idea to design semi-automatic firearms when he noticed while target shooting with friends that grass in the nearby vicinity would blow back from the energy produced from shooting guns. He then started wondering how that energy could be used to cycle ammunition continuously. 

The FN M1900 as designed by Browning was the first pistol to use a slide like virtually all semi-automatic pistols do today. Browning also invented the round to be used in that pistol, the .32 ACP (or 7.65mm). The wide success of the FN M1900 meant that many other handgun manufacturers likewise began to produce pistols in the .32 ACP cartridge as well. 

The M1900 was effectively the start of a new era that continues to this day. Browning went on to design several more highly successful semi-automatic pistols that expanded upon and refined his original design for the FN M1900.

These pistols included the diminutive FN M1905 pocket and Colt M1908 Vest Pocket pistols, the FN M1903 and M1910 pistols, the Colt M1900 and M1902 pistols, and the legendary Colt M1911 pistol.

Browning designed a variety of cartridges for use in these pistols, ranging from the tiny .25 ACP to the big .45 ACP. Speaking of which…

Common ACP Calibers

Common ACP Calibers

Pistol calibers designated as ACP remain popular with the military, law enforcement units, and civilian sport sooters to this day. They include the .25 ACP, .32 ACP, .38 ACP, .380 ACP, and the .45 ACP.

Let’s dive into each of these calibers in greater detail. 

.25 ACP

.25 ACP

The diminutive .25 ACP is the smallest of the ACP calibers. It was originally designed in 1905 and was designed to be used in very small and easily concealable pocket pistols such as the FN M1905 Pocket and the Colt M1908 Vest Pocket.

The .25 ACP was designed to compete directly with the rimfire .22 Long Rifle (LR) round. The .22 LR was likewise being chambered in very small pocket pistols due to its small size, but it suffered from one major flaw: it was not always reliable when used in such pistols. 

The .25 ACP is a centerfire round only slightly larger than the .22 LR, but it was designed to function more reliably in a small pistol.

The main advantage to the .25 ACP is that it can be loaded into some of the smallest and most easily concealable pistols that have ever been produced. The Baby Browning and Colt Pocket Pistols, for instance, are smaller than most pocket pistols produced today. 

.32 ACP

.32 ACP

Released in 1899 for the FN M1900 pistol, the .32 ACP would end up becoming one of the most commonly-used pistol calibers of all time. In the years after the success of the M1900, many other handgun manufacturers began producing their own pistols in the caliber. The most famous example is probably the Walther PPK pistol, due to its association with Adolf Hitler and James Bond.

The .32 ACP still remains in wide use to this very day. It’s an effective round to use for close quarters self-defense, and it’s a very pleasant shooting round to shoot due to the soft recoil it generates. Whereas most compact pistols can be very snappy, most compact pistols chambered in the .32 ACP cartridge are very soft shooting by comparison. 

.38 ACP

.38 ACP

.38 ACP was released in 1900 for the Colt M1900 and M1902 pistols. The M1900 was the first pistol that utilized a full length slide that covered up the entire length of the barrel. And while the M1900 (and its successor the M1902) were undeniably revolutionary, the .38 ACP cartridge itself was not. 

That was because the M1900 and M1902 were largely overshadowed by the Colt M1911 pistol that Browning designed around a decade later. That gun was chambered in the bigger .45 ACP, which was a much more powerful round and considered a ‘man stopper.’ And for smaller handguns and calibers, people mainly stuck to .32 ACP and .380 ACP. 

It’s for this reason that the .38 ACP is the one ACP round that is rarely used today. 

.380 ACP

.380 ACP

Not to be confused with the .38 ACP like we just talked about, the .380 ACP (also known as the 9mm Kurz or 9mm Short), was actually derived from the earlier .38 ACP round. The .380 ACP was designed from the start to be a rimless cartridge, to help aid in accuracy. 

The .380 ACP was also intended from the start for use in blowback pistols that lacked a barrel lock. Blowback operated pistols are often much simpler in design; famous examples include the Walther PPK, Beretta Cheetah, Bersa Thunder, CZ82/83, SIG Sauer P230/P232, and Makarov pistols.

Producing more energy than the .32 ACP at the cost of slightly greater recoil, the .380 ACP is very popular in modern-day pocket pistols, such as the Ruger LCP and the Smith & Wesson Bodyguard. 

.45 ACP

.45 ACP

The .45 ACP is undeniably the most famous of the ACP calibers. Released in 1905, the .45 ACP was designed from the beginning to be a military round. The United States army at the time was fielding .38 Long Colt revolvers, which proved to deliver underwhelming stopping power in combat. 

The military subsequently designed to switch from .38 revolvers to a new .45 caliber semi-automatic pistol. Long story short, Browning’s Colt M1911 pistol in the .45 ACP caliber was the result.

The .45 ACP remains enormously popular today (especially in the United States) for combat, tactical, and sporting use, and a great myriad of pistols are produced in the caliber. It has developed a reputation for delivering excellent stopping power, while also being widely available and easy to procure. 



One final note worth mentioning is that the aforementioned calibers designated as ‘ACP’ can often go by different names. For example, .45 ACP is also often known as .45 Auto, and .32 ACP is also commonly called .32 Auto as well. 

Complicating things a little further is that there are calibers with the designation ‘Auto’ that do not have the interchangeable ‘ACP’ designation. An example here is the 10mm Auto cartridge, which arrived well after Browning’s time. 

It can be a little confusing, but hopefully this article has clarified for you about what the ACP calibers are and how they came into existence. At the end of the day, the ACP calibers like the ones we’ve talked about above have been here for a long time and they’re going to be around for a long time more.  


What is the Difference Between .45 Auto and .45 ACP?

There is no difference between .45 Auto and .45 ACP; they refer to the same cartridge. “.45 Auto” and “.45 ACP” (Automatic Colt Pistol) are interchangeable terms for a specific type of handgun ammunition developed by John Browning and introduced in 1905.

What Does ACP Stand for in Guns?

ACP stands for Automatic Colt Pistol. This abbreviation is commonly used to designate specific calibers of ammunition that were originally designed for use in Colt’s semi-automatic pistols, underscoring their association with one of the firearm industry’s iconic manufacturers.

What Does .45 ACP Stand for?

45 ACP stands for .45 Automatic Colt Pistol. It refers to a handgun cartridge designed by John Browning in 1905 for use in his prototype Colt semi-automatic .45 pistol, which became the renowned M1911 pistol, widely used in the military, law enforcement, and civilian markets.

Is .45 ACP More Powerful Than 9mm?

The .45 ACP typically delivers more stopping power than the 9mm due to its larger bullet diameter and mass. While the 9mm offers higher magazine capacity and lower recoil, the .45 ACP is often favored for its enhanced terminal ballistics, making it a popular choice for self-defense and military use.

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