Those acquainted with the AR platform outside of Call of Duty and Fortnite would also know the AR-15 as the ArmaLite Rifle 15. The AR-9, on the other hand is a variation of the same AR platform but chambered in 9mm which we all know and love. The AR-15 and AR-9 are not the same. The AR-15 & AR-9 are different beasts that can be built to serve a variety of different purposes and roles. Today we explore those differences and evaluate the benefits from the direction you take for your next build.

What is an AR-9?

The AR-9 utilizes 9×19mm Parabellum while maintaining the layout of the familiar and recognizable AR design. Parts such as handguards, trigger packs, sights, and lower components are all compatible and interchangeable between the AR-15 and AR-9. While the AR is known for its unparalleled modularity and vast aftermarket interchangeable parts, those accommodations begin to shift away from the 223 Remington/5.56 NATO pattern meaning there are components that are exclusive to the AR-9.

Parts that are exclusive to the AR-9 include the following:

• AR Barrel for 9×19mm
• BCG for 9×19mm
• Lower receiver for 9×19mm
• Padded buffer tube
• AR-9 Ejector

Why build an AR-9?

The AR-9 uses a pistol caliber through a design originally intended for 5.56 NATO, meaning it’s more than well-suited to manage recoil; it uses a gas blowback system. Since it’s not a direct impingement system, the AR-9 has no need for a gas tube or gas block which saves space within the handguard too. The energy from the propellant burning within the breech is enough to send the bolt backwards and chamber a new round as it returns to battery. The AR-9 bolt carrier, however, can be accommodated by the standard AR-15 buffer tube and recoil spring.

If you’re looking for a short, compact and carbine style AR the AR-9 lower is a great option as a starting point. With its light weight and ability to put a capable pistol round through a personal defense weapon package you could use a barrel much shorter than typical rifle barrels along with a pistol brace. Additionally, Glock magazines can be used in our 80 percent AR9 lowers without the need for a magwell conversion kit unless you’re using an AR-15 lower receiver as your build’s base.

While barrels for 9mm can be as short as only a few inches, if you want optimal terminal ballistic performance it’s recommended to have at least a 12” barrel. In terms of muzzle velocity you can expect to get projectiles flying up to 1,200 feet per second from an AR-9. Between competition shooting, plinking and personal defense this may suit your needs perfectly. Coupled with a suppressor device and subsonic ammunition, you’ll have a fantastic shooting experience not far behind the same package you get in a .300 Blackout; but at a more affordable price.

AR9 vs AR15 Ammunition

9mm is as common as pistol calibers could possibly come, and with overwhelming demand follows a shortened supply (and means to reload said rounds). In today’s market, having the ammo to shoot your guns can be uncertain or at the very least it will cost you a pretty penny. That being said, it’s certainly a lot cheaper to shoot 9mm than it is for any AR-15 caliber. As of 9/29/2021, common factory loads for 9mm can be found for about $0.36 per round while the average 5.56mm or .223 round will easily cost at least $0.60 per round.

What is an AR-15?

The AR15 can be considered the Honda Civic of the rifle world, a reliable and austere design as they come. Developed and scaled down from Eugene Stoner’s AR-10, the standard AR-15 comes chambered in 223 Wylde (capable of dispensing 223 Remington/5.56 NATO), .223 Remington or 5.56 NATO and can use either a gas piston or direct impingement system to cycle rounds.

AR-15s are one of the most common rifle platforms today, the market for parts and components is massive and the quickest to recover from shortages (something that cannot be said for AR-9s). Any builds or replacements that you plan for an AR-15 won’t be limited by stock availability unless you’re very brand specific.

The capabilities between an AR-9 and an AR-15 in making a clean, ethical kill when hunting are vast especially for anything more than small game. Moreover, M193 (5.56 round) is a superior round when used against soft targets compared to 9mm Luger. This is in part to the typical M193 55 grain bullet's velocity and ability to tumble within the target which causes devastating damage.

Projectiles dispensed by an AR-15 will have more speed than an AR-9 with an exit muzzle velocity of about 3,100 feet per second. This is a vital metric specially when it is your desire that your target is downed in preferably as few hits as possible.

Should I get an AR-15 or AR-9?

As the common answer for the question of “What gun should I get” first begins with another question of “What’s the gun for?” You’ll have to think about the intended purpose for this new AR. Below is a list of priorities for both the AR-15 and AR-9 so choose the one that's most relevant for you.

Reasons to get an AR-9 

• Close quarters maneuverability, great for self-defense and competition alike
• Lower cost in components and ammunition
• Less recoil
• Lightweight
• Potential suppression quietness
• Glock magazine compatibility

Reasons to get an AR-15

• More stopping power
• Terminal ballistics (more devastating wound cavity/channel) 
• Short to long distance engagements
• Accuracy
• Wide range of parts compatibility
• Hunting ability
• 556/223 ammo availability and compatibility

Still can’t decide between an AR-9 and an AR-15?

To get the best of both worlds you could also start with an AR-15 as your first platform. Later on, you could build an AR-9 upper to convert the AR-15 to an AR-9 using the separate upper and additional spare parts whenever it’s convenient.

When converting your AR-15 to an AR-9, don’t forget to replace the stock with a pistol brace unless you want to upset your nearest ATF field agent in case the rifle does not meet the overall weapon length requirement for a rifle as typically is the case for an AR-9 build. Interested in reading more on AR pistol braces for your new build? Check out our previous article on SB Tactical’s various pistol braces and the journey of pistol braces in ATF regulations.

Ready to build your next AR-9 or AR-15 with Modulus Arms?

Check out our selection of upper receiver parts and other parts and accessories for your new build. If you’re interested in making the build even more personal then try our flagship Router Jig Extreme to mill out an 80% lower receiver for an AR lower in either caliber (AR-15 or AR-9).