Is There a Difference Between .45 ACP and .45 Auto?


If you’re looking at two items with different names, you’d think you’d be looking at two different products; and most of the time, you’d be correct. However, with the glaring similarities between .45 ACP and .45 Auto, many people wonder if they’re actually the same cartridge. 

There’s no difference between .45 ACP and .45 Auto besides the name; They’re the same cartridge. When the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) took over, the name was changed to avoid trademark issues. This resulted in the same cartridge having two names. 

This article will take a closer look at .45 ACP or .45 Auto in a little more detail.

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What Does ACP Stand for in .45 ACP?

ACP in .45 ACP stands for Automatic Colt Pistol. It was the name given to the cartridges created by John Browning for semi-automatic pistols. Compared to the name .45 ACP, .45 Auto seems to be a shorter version of the previous name. 

Having identical cartridges with different names can be confusing, but with the breakdown of what ACP means, we can see the similarities even in the name. During their work to standardize the firearm industry, the SAAMI wanted to avoid any issues with trademarked names. 

This led to the .45 ACP being renamed as the .45 Auto. So, we now have the same cartridge with two different names. It’s common for people to think that these are two different things. In fact, people have even started their preference for one or the other, but they are the same cartridge with two names. 

This dual name is not unique to the .45 ACP. In fact, many other cartridges are known by more than one name. It can be confusing when ordering ammo if you see another name for something you’re meant to order. For example, the .30-30 Winchester is also known as the .30 Winchester Center Fire.

These different names aren’t a big deal until your friend recommends .45 ACP, but you can only find .45 Auto. During a time like this, it’s vital to know that sometimes there are other names for what you’re looking for. 

Can You Shoot .45 ACP in a .45 GAP?

You can’t shoot .45 ACP in a .45 GAP despite the similarities. They have the same diameter, but the .45 GAP is slightly shorter. The goal in creating the .45 GAP was to create something that would have the same power as the .45 ACP but shorter so it could fit in a handgun. 

Because .45 GAPs or Glock Auto Pistols are slightly smaller, they operate at a higher pressure. Specifically, the .45 GAP operates at a pressure of 23,000 PSI (158.5 kPa) compared to the 21,000 PSI (145 kPa) of the .45 ACP. The different sizes and higher pressure make .45 ACP not work in a .45 GAP. 

The .45 GAP was created about a century after the .45 ACP, but because of its size, it can’t be used with the .45 ACP. The maximum bullet weight of the .45 GAP is 200 grains (13 g). The bullet weight for a .45 ACP is 230 grains (15 g) which is another reason why it won’t work in a .45 GAP. 

Because the .45 GAP is slightly shorter than the .45 ACP, there wouldn’t be enough room in the cylinder for the .45 ACP to fit and the cylinder to close. So, it would be impossible to even try shooting .45 ACP in a .45 GAP without altering the cylinder to allow it to fit. While this may be possible, it’s definitely not recommended as it can risk the integrity of the gun. 

.45 Auto Rim vs .45 ACP

During WWI, the military had selected the Government Model .45 to be the standard weapon used during the war. But they were struggling to arm everyone with this firearm. Instead, an engineer decided to use .45 ACP cartridges in the Colt New Service with the Smith & Wesson Second Model Hand Ejector. 

Together this became what we now know as the model 1917, but there was a flaw in the plan. Even though it did work most of the time, this setup caused cartridges not to be properly ejected by the ejector star. They had to be knocked out or picked out to keep using the gun. 

Half-moon clips became a popular solution as they added the extra rim that was needed for the .45 ACP cartridge to work properly. These moon clips were add-ons that were attached to the revolver separately, but the revolver had to be designed to use these clips for them to work properly. 

The moon clips take up space in the cylinder that would normally be reserved for the rim that .45 ACP cartridges lack. So, if the revolver is not made to be able to handle moon clips, it won’t fit. 

As a temporary replacement, half-moon clips were used, but a few years later, they weren’t the only alternative. In 1920, the .45 Auto Rim was created to allow the ejection of the cartridge without any issues. The .45 Auto Rim was the same makeup as a .45 ACP, but it featured a rim that would allow it to be ejected properly from a revolver. 

While the .45 Auto Rim cartridges were popular and a real problem-solver, replacing your cartridges can be a pain. So, many people prefer to use the half-moon clips as they allow you to use .45 ACP ammo as well as ordinary revolver ammo. Moon clips have become so popular with revolvers that .45 Auto Rim cartridges aren’t really necessary. 

Can You Shoot .45 Auto Rim in A .45 Long Colt?

You can’t shoot .45 Auto Rim in a .45 Long Colt because the Auto Rims are too long to be used with the Long Colt. While they would fit, you wouldn’t be able to close the cylinder to fire. Without the rims, they would fit, but the rims add too much length. 

In theory, your .45 Long Colt could be altered to be able to hold this ammo. You could shave down the back of the cylinder to provide more space, but just like we discussed above, that is not recommended. However, it’s possible if you really want to use .45 Auto Rim ammo. 

Keep in mind that if you try this method, you’ll no longer be able to use ordinary .45 ammo because it will no longer fit. This will create extra space in the case. So, the only way you can use .45 Auto Rim in your .45 Long Colt is to shave off part of the cylinder making other ammo unusable. 

Can I Shoot .45 ACP in My Taurus Judge?

It’s not recommended to shoot .45 ACP in a Taurus Judge, according to the manual. It goes on to state that substituting unapproved ammunition can cause injury or even death. However, .45 ACP ammo can work in a Taurus Judge with the right moon clip.

Please remember, just because it’s possible, doesn’t mean it’s safe. In these situations, it’s highly recommended that you follow the directions laid out in the manual to ensure no injury occurs due to misuse. 

There are videos available of people shooting .45 ACP in their Taurus Judge. So, we know it’s possible. But try this at your own risk, given that it goes against the gun manual. 

If you’re looking for something similar that can be used with .45 ACP, The Smith & Wesson Governor is approved for use with .45 ACP. The Governor comes with a moon clip that allows you to use .45 ACP safely. So, if you’re looking for another option that is safer than using your .45 ACP with your Taurus Judge, the Smith & Wesson Governor is more recommended for use. 

Can .45 ACP Be Used in a Revolver?

Because a .45 ACP is a rimless cartridge, it generally can’t be used with revolvers without a half-moon clip. So, it can be used with revolvers, but only with additions that help it work properly. 

Like we discussed above, the .45 Auto rim was created because .45 ACP did not work with a revolver. The rim allowed the cartridge to fit properly to be ejected with the star ejector rather than being picked out by hand. 

So, this tells us that ordinary .45 ACP ammo won’t work by itself without an adjustment being made. The .45 Auto Rim was an adjustment made to help this, but now most people use half-moon clips. 

Full and half-moon clips are meant to replace the rims that are missing in .45 ACP cartridges. They set the proper headspace for these cartridges to be used. Otherwise, a .45 ACP wouldn’t work with a revolver. 

If you plan to use .45 ACP with a revolver, you need to make sure it’s compatible with moon clips. If it isn’t, then you won’t be able to use your ammo with the revolver.

The reason that the revolver has to be made to use with moon clips is that they require extra space in the cylinder. The cylinder has to be cut so that moon clips can fit in there. 

Moon clips sit on the chamber without changing the headspace. This means that a revolver made to use with moon clips can also be used without them. So, a revolver that can be used with moon clips doesn’t always have to be. This allows you to use your .45 ACP ammo, among others.

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How Does the .45 ACP Compare to Other .45 Cartridges?

M1911 with 45ACP Ammo

While not universal, many other .45 cartridges are shortened to comply with laws regulating military-caliber weapons. So, you’ll find that many other .45 cartridges are smaller to be more widely available. 

The .45 GAP, as we’ve already discussed, follows this pattern. However, they aren’t the only cartridges. The .45 Short Auto is another example of this. The creators took the .45 ACP and shortened it by one millimeter (0.03 in), which allowed it to no longer be classified as a military-grade cartridge. 

Curiously, taking a millimeter (0.03 in) off a bullet allows it to be within standards of a law, as this doesn’t change anything else but the length. But to follow laws regulating military weapons, the .45 short Auto was an acceptable alternative. 

The .45 Schofield is another member of the .45 group. The .45 Schofield has a larger rim than the .45 Colt, and it’s easier to shoot more accurately because of its reduced recoil. The military wanted a revolver that could fully open. Because the .45 Colt did not come out fully during the reload process, it would take longer to load. The .45 Schofield was what they needed. 

The .45 ammo group is quite vast, and these are just a few of the notable cartridges and how they compare to the .45 ACP. But as long as you know about the main comparisons we talked about above, you’ll be able to understand the differences in sizes among the group. 

Remember that gun manuals always list the appropriate ammo to use with the gun you have. If you’re using ammo that is not listed, then you’re putting your safety and the durability of the gun at risk. Keep this in mind whenever you consider modifying a gun to suit your needs.

Final Thoughts

The .45 ACP and .45 Auto may be the exact same ammo under different names. Knowing that they’re the same can make online cartridge shopping a lot easier, but it’s also a fun fact to bring up with friends. 

In general, I recommend continuing research on this topic. As you learn more about the history and nuances of .45 Auto and similar cartridges, you’ll pick up easily on what they’re meant to best used for. For example, if you know that .45 Auto Rim was made to replace the need for moon clips, you’ll understand that .45 Auto cartridges won’t fit into revolvers alone.

Thanks for reading!

For more, check out How Many Rounds in a Box, Brick, and Case? | By Ammo Type.

Jim James

Hey, I'm Jim and the author of this website. I have always been interested in survival, fishing, camping, and anything in nature. In fact, while growing up I spent more time on the water than on land! I am also a best-selling author and have a degree in History, Anthropology, and Music. I hope you find value in the articles on this website. Feel free to contact me if you have any questions or input!

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