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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 80-Lower.com on Sep 1st 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. 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?

A typical 9mm AR Pistol Kit

The AR9 pistol is a 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.


Differences between AR-9 and AR-15

The AR9 and AR-15 certainly look alike. Except the 9mm variant of the AR platform uses some different parts and functions to provide semiautomatic fire. We cover those differences below.

1. 9mm uses blowback to operate.

Look closely at an AR9 upper, and you won’t see a gas tube underneath the handguard. 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.

2. 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 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 in a pistol-caliber firearm like the AR9, all that energy would: 

1.) quickly destroy the bolt and trigger group, or 

2.) not cycle the bolt completely in a blowback-operated AR-15. 

In a rifle-caliber AR, that excess gas must expend some of its energy on the bullet itself as it travels down the barrel, before being redirected to cycle the bolt. This is called dwell time, and it's why gas must first travel down the barrel then back, through a gas tube, before coming back into the receiver. This is also why AR-15s have different gas system lengths. Some rifle rounds and barrel lengths produce less pressure. Some produce more, and require more dwell time to avoid over-pressurizing the bolt.

The 9mm cartridge was designed for handguns, which have no gas system. The 9mm's powder burns rapidly in the firing chamber, and there's simply less of it to burn. So, its energy is harnessed at in the firing chamber, providing the force to cycle the bolt as soon as the cartridge's primer is ignited. 

This is why the AR9 uses blowback: It simply doesn't need a gas system. Instead, the gas the cartridge produces immediately begins actuating the bolt in the chamber. But the AR platform was designed with a delayed-rotation, locking bolt. That brings us to describing which parts are modified for use in the 9mm AR variant.


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

Bolt carrier group

Since the AR-15 relies on a gas system, its bolt must remain locked in the firing chamber until sufficient gas pressure is built to rotate and cycle the bolt. But with 9mm's blowback operation, this wouldn't work. So, the 9mm bolt ditches the rotating, two-piece assembly that rifle cartridges rely on. Instead, the carrier and bolt are a single piece (pictured above).

AR9 and AR-15 bolt carrier group parts are not interchangeable or compatible with each other. The 9mm bolt is physically 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 80% lower) accept Glock magazines. The AR-15 rifle's lower can be made to function with 9mm cartridges, but this requires the installation of a conversion block. Different AR-15 conversion blocks will accept different types of 9mm mags. Options are available that work with Colt, UZI, or Glock magazines. 

Unlike the AR-15, the 9mm AR's bolt does not contain the ejector to remove spent shell casings. Instead, the ejector is attached to the lower receiver, just behind the mag well, as seen above. 

Barrel

The AR9 and AR-15 barrel use the same external dimensions for the barrel extension, so they can be installed interchangeably on any AR-15 stripped upper receiver. 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 sealed to the firing chamber in an AR9 is the spring and buffer inside the buffer tube.


What parts do the AR9 and AR-15 share?

Stripped 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 (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. In addition to the standard LPK, the 9mm lower receiver requires the use of an ejector, like described above. The ejector is typically sold separate from the LPK, often included and preinstalled with the receiver itself.

Buffer assembly

Because the AR9 uses an AR-15 upper receiver, it also usea an AR-15 buffer system. The extra energy produced by 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 plays a big role in determining whether the rounds you're firing stay subsonic or go supersonic. Do you want a close-range AR9 that can be suppressed? Or do you prefer a medium-range, supersonic AR9 that can hit targets at least 100 to 200 meters out? Or, do you want to aim for somewhere in between, where you chamber both supers and subs? Let's break it all down.

Barrel length and velocity

The magic number for making a bullet go supersonic is 1,125 feet per second (FPS). Any bullet traveling faster than that will go supersonic, producing that classic "crack and echo" downrange. A round traveling below 1,125 FPS won't will remain subsonic. The only noise made will come from the gasses leaving the muzzle, which can then be mitigated with a suppressor.

9MM ballistics data

Use this graph to decide what barrel length is best for your 9mm AR 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 www.ballisticsbytheinch.com for collecting this data.

The best barrel length for AR9

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

Averaged across all loads, the optimal barrel length for 9mm is 7" to 9". At these lengths, adding more barrel will only provide an additional 15 FPS for most cartridges, which isn't worth the extra weight. Those who shoot suppressed tend to chamber heavier (135- or 147-grain) loads. To keep these subsonic, you need a barrel that's 5" or less.

The best twist rate for 9mm

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 will handle all 9mm loads.

The correct 9mm buffer weight

Most shooters find a heavy "Rifle" or H3 buffer (5.0 to 5.4 ounces) works best for subsonic, suppressed 9mm loads. A pistol buffer (8.5 ounces) paired with a standard AR-15 recoil spring and buffer tube will get your AR9 operating and cycling reliably for all loads. 

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 AR9 magazines

Most AR9s and 9mm lower receivers produced today accept GLOCK magazines, though few still accept Colt's 9mm SMG magazines. Grab the G-9 Hybrid bolt and a Glock-compatible AR9 lower to use GLOCK mags. The G-9/Glock route is preferred by most builders.

Picking the right 9mm bolt

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 of the bolt's body, near the strike face 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) are ramped.

The upper receiver

Like we said, the AR9 uses a regular AR-15 stripped upper. You can even save some money by investing in a "slick-side" upper with no forward assist. The 9mm bolt can't use the forward assist, anyway. 

9mm handguard compatibility

Since the 9mm AR uses a receiver upper receiver and barrel extension, you get to stick with standard handguards and their included barrel nuts.

You now have all the information you need to build a reliable, accurate AR9!


Why not 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.


Summary

That was a ton of info. Here's a quickly recap.

The 9mm AR uses a modified lower receiver that accepts Glock magazines. 

The AR9 uses a one-piece bolt with the ejector moved to the lower receiver itself.

The AR9 uses blowback operation, ditching the AR-15's gas tube and gas block.

The AR9 uses the same upper, lower parts kit, handguard, and buffer system as the AR-15. 

You'll need a 9mm barrel, and you don't need a gas system. since the AR9 is blowback-operated.

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 80-lower.com, 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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