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AR9 Build & Ballistics Guide

AR9 Build & Ballistics Guide

Posted by Gun Builders Depot on May 13th 2020

The AR-15 platform is no longer reserved for the typical 5.56 NATO and .223 Remington cartridges. With new developments by major manufacturers like PSA and Anderson, the 9mm pistol-caliber carbine ("PCC") dubbed the AR9 has become a popular alternative to the typical black rifle. This blowback-operated variant ditches the gas system and some other important parts are modified. If you want to master this new caliber and build another AR to shoot it, keep reading. Here's your AR9 Build and Ballistics Guide.

What is The AR9?

The AR9 is similar to the AR-15 in overall look and feel, but functionally it is quite different. Like we said, the AR9 operates with blowback. That means the gas produced by the 9mm cartridge forces the bolt carrier group backwards directly. Gas doesn't travel down a gas tube and into the carrier like a normal AR-15.

That means your AR9 build will sport no gas tube and no gas block. The barreled upper you'll be building or buying must also accommodate a different bolt carrier group to function with this new method of gas operation.

AR9 vs. AR-15 Parts: What's Different?

These AR9 parts are different from the AR-15's parts.

Bolt Carrier Groups

Since there is no gas tube required to cycle 9mm rounds, the AR9's BCG (pictured right) doesn't have a gas key. Instead, a "sled" replaces the gas key, which serves to keep the bolt aligned with the lower receiver and barrel inside the upper receiver. AR9 and AR-15 bolt carrier group parts are not interchangeable or compatible with each other.

Lower Receivers

he AR-15 lower receiver is capable of being converted to fire 9mm rounds. That requires the use of a device called a box conversion. A box conversion consists of a metal device that rests inside the magazine well, which retro-fits the AR-15 lower to accept 9mm handgun magazines. Today,  the dedicated AR9 lower instead uses a custom mag well.

Barrel and Barrel Extension

Obviously, the AR9 uses a different barrel and barrel extension than the AR-15. Since blowback operates the bolt, there's no need for a star chamber or gas port on the 9mm barrel. The face of the extension is flat.

AR9 vs. AR-15: What Parts are The Same?

Many parts are still interchangeable between the AR9 and AR-15. This makes  building one easy.

Upper Receiver

The AR9 still uses a stripped AR-15 upper receiver. This is why the AR9 BCG needs a sled where the gas key normally resides: It must keep the bolt carrier aligned with the hammer and barrel. Most AR9 bolts do not have serrations for using the forward assist, so a "slick-side" upper may be used without losing any functionality.

Lower Parts Kit

Buffer assembly

Lastly, the AR9 also uses a regular AR-15 buttstock and recoil spring. The only difference is the buffer itself: To handle the additional force created by blowback gas operation, a heavier buffer is required. Most buffers designated for 9mm are advertised as simply "9mm buffers" or "pistol buffers". If you're shopping by weight, you'll need a 5.5-ounce buffer. Dimensionally, the 9mm buffer is physically the same as an AR-15 buffer.

AR9 Ballistics and Barrel Length

Building an AR9 is quite difference from building an AR-15 not because of the different parts, but because of ballistics. While 5.56 NATO and .223 Remington are exclusively supersonic cartridges, 9mm perfectly toes the line between supersonic and subsonic ammo. And that means your AR9 can function as one of three things:

  • A high-velocity shooter for maximum distance
  • A subsonic shooter with a suppressor for quiet shooting
  • A rifle or pistol that can shoot both supersonic and subsonic

With the right barrel length, your AR9 can reliably reach out and touch targets at 200 yards or more. Or, you can still enjoy high accuracy by sticking with a shorter barrel which provides subsonic performance. So, which one will you pick? To find the answer, we need some ballistic data.

NOTE: You can only build an AR9 with a barrel shorter than 16" if it's configured as a possible. An AR9 with a barrel shorter than 16" and a buttstock equipped is classified as an SBR (short-barreled rifle) by the ATF. 

Subsonic and Supersonic Velocity

Supersonic velocity is achieved at 1,125 FPS. Supersonic cartridges report that classic "crack" as the bullet breaks the sound barrier. So, if you want to build an AR9  designed for long-range shooting, you'll want a barrel that affords a velocity higher than 1,125 FPS. We'll find the max velocity below. 

If you want to build an AR9 that can occasionally shoot supersonic but you still want subsonic options, there will be a "Goldilocks zone" that provides velocities both above and below 1,125 FPS. And lastly, if you want to build a compact close-quarters-type AR9 that only ever shoots subsonic, you'll want the shortest possible barrel.

Lets look at the ballistic graphic below and determine what velocities (and thus, what barrel lengths) are best for all three options. We'll use Federal 115-grain JHPs for this example since 115-grain bullets -- and Federal ammo in general -- are some of the most common 9mm cartridges.

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

Best Barrel for Supersonic Performance

Looking down the column for Federal 115-grainers, we can see that velocity effectively maxes out with a 12" barrel at 1,282 FPS. If you go any longer, you'll only gain inside 10 to 20 FPS. In fact, that same logic seems to hold true across all types of 9mm ammo. For lightweight 90-grain and heavy 147-grain bullets, negligable gains in velocity are achieved beyond a 12" barrel. If you're going fexclusively supersonic for maximum-distance shooting, a 12" barrel's the best choice for your AR9.

Best Barrel for Supers and Subs

If you want an AR9 barrel to perform well with both supersonic and subsonic ammo, you'll need to switch up bullet weights. Lightweight rounds go supersonic, and heavy loads go subsonic. If we want to get the heaviest 9mm ammo (147-grain) traveling as close to supersonic as possible for maximum power, and we want the shortest possible barrel, we would want to stick with a 5" barrel. 

With a 5" barrel, 147-grain loads will approach the 1,115 FPS point, which is 10 FPS below supersonic. This barrel length provides the most stability and distance without producing supersonic noise. 

Best Barrel for Subsonic Only

This is a little counter-intuitive: If you're building an AR9 and plan on shooting only subsonic ammunition, then you'll only be shooting 147-grains. It simply provides no benefit to shoot lighter bullets while attempting to remain subsonic. That means you can technically pick any barrel length. Why? Because virtually all 147-grain 9mm ammo remains subsonic, even when pressure is maximized with a long 18" barrel. What's more, if you cut down that 18" barrel to just 5", you lose less than 80 FPS in velocity. 

The Best Twist Rate

This one's simple: A 1:10 twist rate is the best twist rate for 9mm Parabellum. There is no "better" twist rate. Through decades of ballistic testing and hundreds of hours of special operations, SWAT, FBI, counterterrorism, and other agencies testing and fielding 9mm carbines, the 1:10 twist rate was found to provide the best rate of spin and the most stability for any 9mm cartridge.

Building Your AR9: Magazine and Bolt Compatibility

There are two types of magazines used for AR9s. Glock and Colt. Picking one or the other will dictate what type of bolt carrier group you buy, too. Let's look at magazines first. In our humble and professional opinion, you should stick with Glock magazines. Sure, Colt magazines are cool because they were used in submachineguns and 80's action movies often showed them off. But modern Glock magazines are way more affordable and reliable. There are more options to pick from, and they have a higher capacity. Most AR9 lower receivers and uppers are made to work with Glock magazines, too. That brings us to the bolt discussion.

Picking Your AR9 Bolt

The G-9 Hybrid bolt is the only bolt guaranteed to be compatible with Glock magazines. Other bolts may be available from other manufacturers which will advertise compatibility with both Glock and Colt magazines. Another important part of the BCG is the underside:

The portion of the bolt carrier circled in red is where the hammer interacts with the carrier. This section of the carrier must be slanted, or "ramped", to allow for compatibility with an AR-15 or M16 hammer and parts kit. If your bolt carrier isn't ramped, you'll need to use an RRA 9mm hammer, which is cut differently.

Frequently Asked Questions

Why not convert my AR-15 to 9mm?

It's true that the 9mm-converted AR-15 existed long before the dedicated AR9. Box magazines have been around for years. So too have 9mm barrels with AR-15-type extensions. Yet these box magazines are not the way to go for a few reasons:

1. Box magazine conversions can be clunky, expensive, and unreliable. Small variances in receiver magazine well dimensions could cause problems when it comes time to load 9mm. Often, box conversions are loose and wobble inside the receiver. Truthfully, it looks a little silly to see a skinny 9mm magazine resting inside a 5.56 NATO/.223 mag well, too.

2. Pulling the barrel and barrel nut on a used AR-15 can damage the upper receiver. Over time, the barrel nut and barrel extension tends to seize to the receiver and often, heat and excess torque are required to break the barrel nut loose. 

3. Chances are, the AR-15 you'd like to convert was built as a rifle. Once a firearm is configured as a rifle, it must stay a rifle. You legally cannot install a barrel shorter than 16" on that firearm and make it a pistol. The ATF will instead reclassify the weapon as a short-barreled rifle, which could land you in prison without the proper NFA paperwork and tax stamp. That means you'd need to install a 16" barrel, which at least partially defeats the purpose of building a 9mm AR. After all, enjoying a caliber that performs so well with a much shorter barrel is one of the great advantages the AR9 holds over the AR-15.

How far can my AR9 shoot?

With a 12" or longer barrel providing maximum velocity, you can reliably strike targets at up to 400 yards with lethal effects. You've probably heard of famous firearm YouTuber Iraqveteran888.  Check out his video performing this very feat with iron sights.

What does an AR9 build kit include?

Build kits  include every part required for assembling your AR9:

  • Barreled upper receiver with railed handguard
  • Billet AR9 80% lower with ejector installed
  • Mil-spec buffer tube with SBA pistol brace
  • Mil-spec recoil spring with AR9 buffer
  • Mil-spec AR-15 lower parts kit

Kits can be bought with or without the 80% lower. Since the AR9 uses the same takedown pins and lower parts kit,  AR-15 jigs and drill bits are almost always compatible with the AR9 80% lower.

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