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

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

Posted by on Sep 18th 2020

Updated March 2022.

This is your ultimate AR9 pistol guide! We’re breaking down everything you need to know to build one yourself: What the AR9 pistol is, what separates it from an AR-15, parts compatibility, 9mm Parabellum's ballistics, buffer weight, barrel length, twist rate, and magazines. When we're through, you'll know everything you need to build the perfect AR9 pistol in a supersonic or subsonic (suppressor-friendly) configuration. Looking for answers about shouldering an AR pistol brace? We wrote a guide for that, too. All data and recommendations in this guide were compiled from real shooters who've built and tested AR9s with different parts.

What is an AR9 pistol?

9mm AR Pistol Kit (which must be paired with a 9mm AR lower) is what many shooters buy to build from scratch.

The AR9 pistol (sometimes hyphenated "AR-9") is a new 9mm variant of the AR-15. But this particular 9mm weapon is not just an AR-15 with a 9mm Parabellum conversion kit installed. Converted 5.56/.223 guns use special parts to adapt the factory upper and lower receivers for the 9mm cartridge. The AR9 pistol instead uses a new lower receiver (aptly called the AR9 lower) and a stripped AR-15 upper receiver with a 9mm barrel, new bolt carrier group, and an AR-15 buffer system and lower parts kit. A 9mm-specific buffer is required inside your buffer tube, but we'll touch on that later. Since the AR9 pistol and AR-15 appear so similar, let’s compare the two first.

How the AR9 is different from the AR-15

The AR9 and AR-15 look alike, but each uses different parts and systems to operate. There’s a lot of discussion – and some confusion – about how these two guns work.

9mm uses blowback to operate

Look closely at a typical AR9 upper, and you won’t see a gas tube anywhere. That’s because the AR9 uses blowback to cycle the bolt. Here’s how it works:

  1. The firing pin hits the primer on the 9mm cartridge.
  2. Powder burns in the firing chamber and creates gas.
  3. The rapid expansion of gas expels the bullet from the barrel.
  4. That same gas forces the spent casing to slam against the bolt inside the chamber.
  5. The kinetic energy of the spent casing drives the bolt back into the buffer tube.
  6. The spent casing is ejected, the bolt drives forward, and a new round is chambered.

The AR-15 uses gas or a piston

The AR-15 uses a direct-impingement system with a gas tube to cycle the bolt:

  1. The firing pin hits the primer on the rifle cartridge.
  2. Powder burns in the chamber, accelerating the round.
  3. Excess gas travels to a gas port and through a tube or piston.
  4. The gas/piston travels back into the upper receiver and bolt carrier group.
  5. A key atop the bolt traps the gas/piston and allows it to drive the bolt into the buffer.
  6. The spent casing is ejected, the bolt drives forward, and a new round is chambered.

Why the AR9 can't use a gas tube

A rifle round has a lot of powder. It produces a lot of gas when fired, and burning all that powder takes time. This is the only way to achieve the right amount of velocity a rifle round requires. But all that round's energy would 1.) quickly destroy the bolt and trigger group, or 2.) not cycle the bolt completely in a blowback-operated AR-15. That gas must expend more of its energy on the bullet itself before being harnessed to cycle the bolt. This is called dwell time, and it's why gas must travel down the barrel and through a gas tube before coming back into the receiver. This is also why AR-15s have different gas system lengths. Some rounds and barrel lengths produce less pressure. Some produce more pressure and require more dwell time to avoid over-pressurizing the receiver.

The 9mm cartridge and its bolt was designed to cycle with blowback recoil, not gas. The 9mm burns nearly all of its powder rapidly in the firing chamber, and there's simply less of it to burn. So, its energy must be harnessed “at the source” to provide the necessary force to cycle the bolt. This is why the AR9 uses blowback. Some builders have attempted to use gas impingement in AR9s and pistol-caliber AR-15s with unreliable results. Using gas impingement with a 9mm cartridge requires an incredibly short gas tube and ultra-light buffer and spring. This dramatically increases felt recoil and adds weight for virtually no good reason. No bueno.

AR9 vs. AR-15 parts: What’s different?

Bolt carrier group

The AR-15 and AR9 bolt are totally different. The AR-15's BCG is two pieces. It uses a carrier and rotating bolt which delays opening to build sufficient pressure. The 9mm bolt is a single piece and does not use a separate, rotating bolt or carrier. It cannot be modified to work with gas impingement. AR9 and AR-15 bolt carrier group parts are not interchangeable or compatible with each other. The 9mm bolt is also shorter and replaces the gas key with a solid piece of steel meant to keep the bolt aligned with the upper receiver and buffer.

Lower receiver

Most AR9 pistol lowers ( like this 9mm AR 80% lower, our most popular choice) take factory and aftermarket Glock or Colt magazines out of the box. The AR-15 rifle lower requires a conversion block to be installed inside the magazine well before it can take 9mm magazines. Different AR-15 conversion blocks will accept different types of 9mm mags. Options are available that work with Colt, UZI, or Glock magazines. Conversion blocks can get expensive. Most blocks cost between $100 and $250, whereas a typical AR9 lower receiver costs just as much, if not less – around $150. Most AR9 lowers (like our 9mm 80% lower) use a proprietary magazine release. Most have the mag release pre-installed, though some may use the AR-15 magazine release included in your parts kit.


The AR9 and AR-15 barrel are similar, but different. They use the same external dimensions for the chamber/breech, so they can be installed on any AR-15 upper. Rifle barrels (5.56, .223, .308, and 300 Blackout) use a conventional star chamber to lock the bolt until gas pressure builds sufficiently to cycle it, whereas 9mm barrels have no star chamber because of blowback operation. The only thing keeping the bolt locked in an AR9 is the spring and buffer.

What parts do the AR9 and AR-15 share?

Upper receiver

Even though the AR9 pistol doesn’t use gas to operate, it shares the same upper receiver with the AR-15. Upper receivers typically come in billet or forged aluminum. There are polymer uppers available for the AR-15, though we recommend against using one for an AR9 build at this time. To our knowledge, the added pressure and energy of the AR9 bolt's blowback operation has not been tested in a polymer upper.

Lower parts kit and trigger

Most AR9 lowers (again, like ours) use a standard AR-15 lower parts kit. Because of this, an upgraded drop-in trigger designed for the AR-15 can also be installed in the AR9.

Buffer assembly

Because the AR9 uses an AR-15 upper, it can also use an AR-15 buffer system. The added kinetic energy from the 9mm’s blowback operation means you'll need a heavier buffer than what is found in a gas-operated AR-15. We'll get to that next!

How to Build an Awesome AR9 Pistol

Now we know how an AR9 is different from (and similar to) an AR-15. We can start picking out parts for a build! The most important factor of your AR9 build is your barrel. Unlike regular ole' black rifles, your AR9's barrel isn't just there to make your pistol as accurate as possible. Your AR9's barrel length will play a big role in determining whether the rounds you're firing stay subsonic or go supersonic. Do you want a close-range, subsonic AR9 that can be suppressed and dead-quiet? Or a medium-range, supersonic AR9 that can hit targets at least 100 meters out? Or do you want to aim for somewhere in between, where you can play with hand loads and velocity? Let's break it all down.

Barrel length and velocity

To figure out the best barrel length, we need to look at the ballistics of the 9mm cartridge and we need to learn a little science. The magic number for making a bullet go supersonic is over 1,125 feet per second (FPS). Any bullet traveling faster than 1,125 FPS will announce that classic "crack and echo" downrange. A round traveling below 1,125 FPS won't break the sound barrier (it remains subsonic). The only noise made will come from the gasses leaving the muzzle (which can then be mitigated with a suppressor). Transonic rounds just barely approach 1,125 FPS. In this velocity gray area, the subsonic sound signature changes and gets louder.

9MM ballistics data

You can use this graph to decide what barrel length is best for your AR9 pistol given your choice of ammo. Federal 115-grain JHPs (center column) are some of the most common 9mm rounds on the market, so let's look at this data as an example.

Full credit goes to the shooters over at for collecting this data.

The best barrel length

A gun barrel's length is considered optimal when adding more length only yields negligible increases in velocity. For nine of the most common 9mm loads, we highlighted this point of diminishing returns in green. That is, where adding more barrel length only provides an additional 15 FPS in velocity. Velocities highlighted in yellow indicate where adding one more inch of barrel decreases velocity, hurting performance.

Averaged across all nine loads, the optimal barrel length for 9mm is, fittingly, 9 inches. For all loads 90 grains up to 147 grains, adding any more barrel will only provide an additional 15 FPS, which simply isn't worth the extra weight and flex. Many pistol calibers can be suppressed with amazing results. Most shooters who shoot suppressed stick to heavier (135- or 147-grain) loads. To keep these subsonic, you need a 5" or shorter barrel.

The best twist rate

Surprisingly, picking the best twist rate for your AR9 pistol is easy. Hundreds of sub machineguns and handguns chambered in 9mm have already helped us to learn that a 1:10 twist rate is preferred. When Colt first introduced the AR9 platform, this is the twist rate they chose. Shooters have tested different twist rates ranging from 1:10 to 1:20. Most report negligible differences in shot placement and accuracy. Unlike rifle rounds (which are long, fast, and require extra help to stabilize), the 9mm is short, slower, and stubby. It also burns most of its powder immediately, which means it reaches optimal velocity quicker and stabilizes more readily with less rifling. 

So, twist rates don't affect the AR9's accuracy as much. The general rule of thumb on barrel twist rates is simple: The heavier the bullet (higher grain count), the more extreme the twist rate. A 1:10 twist rate will handle all 9mm loads, while a less extreme rate (like 1:15 or 1:20) may not stabilize a heavier round (147-grain, for example) as well.

The right buffer setup

Buffer systems have been made specifically for the AR9, but they may cost more than a standard AR-15 buffer kit. Most shooters find a heavy rifle/H3 buffer (5 to 5.4 ounces) or a pistol buffer (5 to 8.5 ounces) paired with a standard AR-15 recoil spring and buffer tube will get your AR9 operating and cycling reliably. We recommend the Shockwave AR9 Pistol Lower Parts Kit (includes a buffer and pistol brace) and adding extra weight as needed to reduce felt recoil. If your AR9 doesn't fully cycle or fails to eject frequently, a lighter buffer may be required.

Picking Glock or Colt magazines

The magazine you choose for your AR9 will determine what bolt and lower receiver you pick. Most AR9 parts will say “Glock-style” and/or “Colt-style”. Builders have reported compatibility issues when mixing and matching Glock and Colt AR9 parts. The rule of thumb is this: Pick a bolt and lower that are compatible with your 9mm magazine of choice. Grab an upper with the G-9 Hybrid bolt and a Glock-compatible AR9 lower if you want to use Glock magazines. The G-9 hybrid/Glock route is recommended and preferred by most builders. If you’re dead-set on using Colt 9mm magazines, stick with a Colt-style AR9 bolt and Colt-style lower.

The right lower parts kit

Look for this ramp/slope underneath your 9mm bolt.

In order for the AR9 to use a regular AR-15 hammer and parts kit, your 9mm bolt needs to be ramped. The ramp is the sloped portion underneath and center, near the head of the firing pin. If your bolt is not ramped, you'll need a modified hammer to address clearance issues between the upper and lower. Most 9mm bolts (including the G-9 Hybrid) are ramped by default.

The right upper receiver

Like we said, this is also an easy pick because the AR9 uses a regular ole' stripped upper. You can choose anyAR-15 upper, but you can even save some money by avoiding the forward assist feature. Their bolts don't use the forward assist. Just like that, you now have all the information and parts data you need to build a reliable, accurate AR9!

Why not just convert an AR-15 lower to 9mm?

Great question! At first glance, converting a rifle to 9mm might seem like an easy thing. All you need to do is buy a conversion block and 9mm barrel, slap in a magazine and pull the trigger, right? Wrong. Chances are, the AR-15 you're converting is set up for 5.56 or .223, so a 9mm bolt carrier group is required. You'll have to swap out your buffer and remove your gas block and tube. Lastly, you'll need to pick a reliable conversion block, and you'll need to make sure the block is compatible with your parts kit, new bolt, upper and lower, and 9mm magazine of choice.

Trust us, building an AR9 is easier. Thanks to the 9mm 80% lower, you can even build yours without going through an FFL and paying extra taxes, dealing with fees and paperwork, waiting 10 days for your purchase, and so on. Plus, you're only buying the parts you want when you build, not some accessories or added money-grabbers at the gun store.

Is there any downside to a 16" 9mm barrel?

Of course not! Well, besides adding some extra weight. Even though you may experience diminishing returns on your "barrel-length-vs-velocity" investment, running up to a 16" barrel (even an 18" barrel) will afford higher velocities and thus, theoretically better accuracy and range. In short, an AR9 rifle will still outperform an AR9 pistol with a sub-16" barrel.


That was a ton of info. Here's the short n' sweet: The AR9 is similar to the AR-15, but different. It uses the same upper receiver, lower parts kit, buffer system, and accessories. The 9mm AR uses a different lower receiver and can use either Glock or Colt magazines (depending on which parts you pick). You'll need a new 9mm barrel, and you don't need a gas system - the AR9 is blowback-operated. You can even build an AR9 with an 80% lower and regular AR-15 jig. Now get to building! Questions about the AR9? Just call us.

DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

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