How to Build a 9mm AR: The Complete AR9 Pistol Guide thumbnail image

How to Build a 9mm AR: The Complete AR9 Pistol Guide

80 Percent Arms   |   Apr 6th 2021

This is the ultimate AR-9 guide that you can use in 2021 and beyond. In this article we've done our best to break down everything you need to know before you build one yourself at home. As we delve deeper into the details, we'll help you understand what an AR9 pistol is and the main features that it comes with.

Once you're done going through this guide you'll know the difference between an AR-9 and AR-15. You’ll also know more about the 9mm Parabellum's ballistics, AR-9 parts such as the buffer system, twist rate, barrel length and magazine compatibility. We share these details with you so that you don't get stuck as you build your wildest AR9, even a suppressor-friendly one.

What Is an AR9 Pistol?

An AR9 pistol is an AR-style carbine rifle that shoots 9mm rounds. Sometimes, the AR9 pistol is spelled as AR-9, but don't pay any mind to the hyphen or lack thereof. This weapon is the latest variation of the AR-15. Most builders will use a 9mm AR pistol kit to to build from scratch. For the quickest "building" experience, simply pair an AR-9 complete upper with an AR lower build to accept 9mm magazines. 

Keep in mind that the AR9 pistol is a 'pistol' because typically the PDW builds are shorter than the federal standard of 16" barrel length — anything shorter than that is classified as a 'SBR' or short barrel rifle unless you use a stabilizing pistol brace. Now, you can buy a 9mm parabellum conversion kit to convert an AR-15 lower receiver to accept 9mm magazines as well. All you need at that point is the AR9 upper to complete the ensemble.

A dedicated AR-9 lower, on the other hand, is unique because it directly accepts Glock-style 9mm magazines. Since it is still utilizes the AR platform, you'll need a 9mm specific buffer system, a new bolt carrier group, and an upper receiver with a barrel chambered in 9mm.

AR15 vs AR9

What’s the Difference Between AR9 and AR-15?

If you just focus on the appearance of the two guns in the image above, you will notice that the two pistols have a striking resemblance. Upon a closer look, they use completely different calibers and parts. The two guns work differently and here is how they are not alike:

1. The 9mm Needs Blowback to Operate

If you have the AR9, take a closer look at its upper receiver, and you'll notice that the gas tube is missing. The reason for this is that the AR9 makes use of blowback to cycle the bolt. Here’s how it works:

-The firing pin comes into contact with the primer found on the 9mm cartridge.
-Powder burns within the firing chamber and then creates a gas.
-Due to the rapid expansion of the gas, the bullet is expelled from the barrel.
-The same gas then forces the casing spent to slam on the bolt within the chamber.
-Kinetic energy then moves the bolt back into the buffer tube.
-The casing is ejected, the bolt moves forward and then a new round is chambered.

2. The AR-15 Uses a Piston or Gas

For a more comprehensive comparison of the AR9 vs AR-15, check out previous blog. Here’s how the AR-15 works:

-The firing pin strikes the primer on the cartridge.
-Powder then burns within the chamber, the accelerators, and the round.
-Excess gas moves to the gas port through a piston or tube.
-The piston/gas rushes back to the upper receiver and bolt carrier group.
-A key on top of the bolt traps the piston/gas and then lets it move the bolt into the buffer.
-Used casing is ejected, the bolt moves forward and the new round is chambered.

Why Can’t the AR-9 Use a Gas Tube?

The short answer is because it doesn't produce the necessary gas pressures to re-direct enough gas to actuate a bolt that way. Here's the long answer:

With the 9mm being a pistol cartridge, it's optimized to perform out of short barrels of around three to five inches in length. Pistols typically achieve this by using faster burning powders than rifles, so they have a very quick spike in gas pressure, which rapidly dissipates over longer length barrels. A direct-impingement system like the AR-15 works by re-routing some of that gas further down the barrel, once the bullet passes a gas-port. 

However, the reason that you typically only see 9mm SMGs or 9mm PCCs in a direct-blowback type system is because the pressures aren't high enough to actually actuate that kind of piston system. This leaves us with a weapon that could potentially recoil a bit harder than other modern systems, but no need to worry about that as there are plenty of work-arounds. Nowadays, many 9mm PCCs will use a radial or roller delayed type system, but our AR-9 build kit sticks to direct blowback.

How to Build a Good AR9 Pistol

Now you understand that while the AR9 and AR-15 look similar, they have significant differences — Let's start selecting the parts to build.

Barrel selection is a very important part of the process for building your gun. The barrel has a lot of factors that affection how accurate and effective your pistol is. Its length will determine whether the fired rounds will fly at  supersonic or subsonic speeds, so you must choose carefully based on your intended use. Ask yourself if you are looking for a close-range, subsonic rifle that can be easily suppressed and be dead quiet or if you need an AR9 that is suitable for engaging medium range targets up to 100 meters away?

Don't feel cornered into picking one route either because it's also possible to choose a barrel length that lets you aim for something in between and that allows you to play with different ammo velocities; especially if you're using custom hand loads. The devil is always in the details and knowing more will help you get closer to deciding which size will best suit your needs.

ar9 pistol build

What Is the Best Barrel Length?

In order to know the best barrel lengths that fully utilize the ballistics of the 9mm cartridge, you also need to learn some of the science behind ballistics. To make your bullets go supersonic, then the best metric to measure against is 1,125 feet per second. Make sure to do your research or if you can afford to experiment with different barrels, to use a chronograph. If your bullet flies faster than that, it will also produce a crack and echoing sounds down range.

However, if the round travels at a speed less than 1,125 feet per second, it will not break the sound barrier, which means it will remain 'subsonic'. The only sound you will hear will be the the gasses leaving the muzzle and your BCG going back and forth.

A maxed out barrel length is determined when adding more length only yields a negligible increase in velocity. Therefore, when choosing the ideal barrel length, you should have some idea in your mind before as to what your ideal preferences are. In layman's terms, it doesn't make sense to increase the barrel length when you will only be decreasing the bullet's speed, potentially making it less effective. 

What Is the Best Twist Rate?

Thankfully, picking the ideal twist rate for your AR9 is one of the easiest things to do. Lots of handguns and rifles that come chambered in 9mm have already proven the fact that 1:10 twist rate is the best.

When the AR9 platform was introduced, 1:10 is the twist rate that the manufacturers settled on. Many shooters tested twist rates that range from 1:10 to 1:20 and the consensus seems to be that there is a negligible difference within the range in terms of accuracy and placement.

Compared to rifle rounds that happen to be fast and long which require additional help to achieve stability, the 9mm is relatively shorter, stubbier and slower. It also gets most of its power burned immediately. These results imply that the pistol attains the optimal velocity much faster and stabilizes more easily without a lot of rifling needed.

The AR9’s accuracy cannot be affected by the twist rate. When it comes to the twist rate, the rule is very simple; the higher the grain count (weight of the bullet), the higher the twist rate. So with a twist rate of 1:10, you can handle a wide range of ammo loads. Note that if your AR9 does not cycle properly or eject spent casings without failure, then you might need a lighter buffer.

What Is the Right Buffer Setup?

Speaking of buffers, which is the correct setup? The buffer systems you'll need for an AR9 build is conveniently the same kind of buffer that you'd use in an AR-15. Based on one our AR-9s we built for marketing purposes we can tell you that in our experience that a. 10.5" barrel, paired with a standard H buffer spring with a 6.5 ounce weight and a muzzle brake makes the AR9 pistol a dream to shoot. The cycling is perfect and so soft you'd think that you were shooting an airsoft gun.

Choosing the Best Colt of Glock Magazines

The type of magazine you choose for your AR-9 will also determine the lower receiver and bolt you go for. Most parts of the rifle will say “Colt-style” or “Glock-style.” Most builders say that some compatibility problems are created when trying to mix and match Colt and Glock AR9 parts, so you should keep that in mind as you’re building your AR-9.

The rule is that you are supposed to choose a lower and bolts that are perfectly compatible with your 9mm magazine. If you intend to use Glock magazines, it's advisable to choose a Glock-compatible AR9 lower and an upper with a G-9 hybrid bolt. Some builders prefer the G-9 Glock/hybrid route. If that's you and you've already made up your mind to use the Colt 9mm magazines then it will be necessary to stick with the Colt-style AR9 bolt and the Colt-style lower.

What Is the Ideal Lower Parts Kit?

For your AR-9 lower parts kit, simply use a standard AR15 LPK because it is completely compatible and both AR platforms use the same parts. you should ensure that the 9mm bolt is ramped. This is the slopped part that is right underneath and that is in the center next to the head of your firing pin. Unless the bolt is ramped, you must use a modified hammer to get rid of the clearance issues between the lower and upper. Thankfully, most 9mm bolts come ramped, so you won’t have to worry about that.

ar9 build

What About the Right Upper Receiver?

This is one of the easy picks since your AR9 leverages the regular pistol’s stripped upper. While you are free to go with any AR-15 upper to save more money, you can neglect the forward assist feature. This is because the pistols come with bolts that don’t use the forward assist, which is why they are not necessary.

Is It Possible to Convert AR-15 to 9mm?

This is a good question. You can easily buy the part to convert an AR-15 lower receiver to accept 9mm Glock mags by using a conversion block and swapping the upper receiver for one with a 9mm barrel. That's the easy way. The more difficult method, but still possible, would be to use an AR15 upper receiver and place an AR-9 BCG in it (obviously still with a barrel chambered in 9mm and your choice of rail/handguard). If you're lucky, everything will work straight after dropping the parts in and putting them together. But sometimes there can be some complications - such as spent casings not properly ejecting since the ejection port is made for .223 and 5.56 (much larger than 9mm). For practicality, it's much easier to simply use a dedicated AR-9 upper and lower receiver do avoid fitment and functionality issues.

Start Your Next AR-9 Build With 80% Arms!

The main takeaway here is that the AR-15 and AR9 share many of the same internal and external parts/components — so make sure to do your homework prior but by now you should know what you'll need for this project. (The AR9 uses the same upper receiver, buffer system, lower parts kit, and accessories). In a future blog, we'll get into the details of the "last round bolt hold open" function" (LRBHO) that you should know about if you plan on buying our AR-9 upper or lowers. So if you're ready to start gathering all the parts to build out your first 80% AR-9 lower, we have everything you need to complete that build! Let our customer service team know if you need any help at any point in the build process along the way and they'll be happy to assist.