Short answer: an AR-9 is an AR-15-style carbine or pistol that shoots 9mm. It has no AR-15 gas system: the bolt and a heavy buffer, or in CMMG's case a bolt that has to rotate before it can cycle, handle the recoil instead. You keep your trigger, grip, safety and stock, and change four things: the magazine well and magazines (Glock-pattern or Colt SMG-pattern), the 9mm bolt carrier group, the 9mm barrel and upper, and the buffer and spring (about 8 oz for straight blowback). Pick your magazine pattern first, because it decides the lower.
What is an AR-9, and what carries over from an AR-15?
"AR-9" means a 9mm AR. The upper and lower look like an AR-15, but the action is a pistol-caliber blowback design, so the parts that touch the cartridge and the recoil change. The trigger group, grip and safety are standard AR-15 parts; our lower assembly guide shows an AR-9 lower built with a drop-in trigger and an ambidextrous selector.
Blowback vs delayed blowback
In a straight blowback gun the only thing keeping the chamber closed is the weight of the bolt and the spring behind it, so a 9mm AR needs a heavy bolt carrier and buffer. Aero Precision builds its EPC-9 receivers "for the extreme demands of pistol caliber blowback ARs." CMMG's radial delayed blowback (RDB) is different: the bolt must rotate and unlock before it can cycle, which slows the action and lets the gun use a lighter bolt and spring. CMMG says the result is a lighter rifle with less felt recoil. Read CMMG's own explanation: What is Radial Delayed Blowback?
- Straight blowback (Aero EPC-9 and most aftermarket AR-9 parts): simple, widely supported, needs the heavier buffer.
- Radial delayed blowback (CMMG Banshee and Resolute lines): lighter, but uses CMMG's own bolt, so you buy that system as a whole.
Glock vs Colt SMG magazines
An AR-9 takes one of two common magazine patterns. The lower or a conversion block decides which.
| Glock-pattern magazines | Colt SMG-pattern magazines | |
|---|---|---|
| What they are | The plastic or steel double-stack magazines Glock 9mm pistols use. | Steel magazines made for the Colt 9mm submachine gun, which is built on the AR-15 layout. |
| Which lowers | Dedicated Glock lowers such as Aero's EPC-9, or an AR-15 lower with a Glock conversion block. | Colt-pattern lowers (CMMG's Mk9) or a block made for that pattern. |
| Why choose them | You may already own the magazines, and they are common and cheap. | Steel SMG-style magazines; CMMG built its original Mk9 around them. |
| Cost in our catalog | From about $11 (Glock 17-round) to $18 (21-round Magpul). | Few stocked; we list a Colt SMG magazine loader at about $24. |
| Last-round bolt hold-open | Not automatic: Aero builds it into its EPC-9 upper receiver; check yours. | Depends on the lower and magazine; check the maker. |
Aero's EPC-9 lower accepts standard-frame Glock magazines and uses the standard AR-15 takedown and pivot pin pattern. If you would rather start from an AR-15 lower, a conversion block gives it a 9mm magwell: Sylvan Arms describes its block as "a drop-in fit to give you a 9mm capable lower." We do not stock dedicated AR-9 lowers, but our lower receivers include multi-caliber AR-15 lowers, and we carry the conversion blocks and magwells below. CMMG sells both a Glock-magwell version (MkGs) and a Colt-magwell version (Mk9) of its pistols.
AR-9 buffer weights
A blowback AR-9 needs a heavier buffer than a gas-operated AR-15. Aero's EPC buffer is about 8 oz and is paired with an AR-15 carbine spring (its kit is titled 7.7 oz and its support page says 8.1 oz). Our AR-15 buffer weights guide lists 9mm blowback builds at 8 to 10 oz and up. CMMG's RDB guns are the exception: CMMG says they need no heavy bolt or stiff spring, so do not apply blowback buffer rules to them.
The parts that change, with today's prices
Lowest price across the stores we track:
Faxon 9mm Glock/Colt bolt carrier group
Pros
- Works with Glock and Colt pattern builds
- Nitride finish
Cons
- Dedicated 9mm part: not an AR-15 BCG
9mm BCG
LBE Unlimited 9mm carbine buffer
Pros
- Buffer made for 9mm builds
- Low price
Cons
- Check the weight against your bolt
9mm buffer
AR-15 magazine quick change adapter block, 9mm
Pros
- Adapts an AR-15 lower to a 9mm magwell
- Keeps your AR-15 lower
Cons
- Check which magazine pattern it takes
Mag block
Magpul PMAG 21 GL9 (Glock-pattern)
Pros
- 21 rounds
- Polymer Glock-pattern magazine
Cons
- Test it in your gun
Glock magazine
Tacfire 10" 9mm complete upper
Pros
- 10" barrel, 1:10 twist
- Complete upper assembly
Cons
- Check magazine pattern and buffer
9mm upper
Browse bolt carrier groups, uppers, barrels, buffers and magazines, and use the AR-15 builder to check the rest. Building a 9mm pistol rather than a carbine? See how to build an AR pistol for the rules on stocks and braces, and our alternate caliber comparison for 9mm against 300 Blackout and 7.62x39. Ammo cost per round is on our ammo cost data page.
Frequently asked questions
What is an AR-9?
A 9mm rifle or pistol built on AR-15 style receivers, using a blowback or delayed-blowback action instead of the AR-15 gas system.
Is an AR-9 blowback or gas operated?
Blowback. Aero's EPC-9 is a straight blowback design, and CMMG's Banshee uses radial delayed blowback. Neither is gas operated like an AR-15.
Glock or Colt magazines for an AR-9?
Glock-pattern magazines are the most common and you may already own them. Colt SMG-pattern magazines suit builds with a Colt magwell. Choose the pattern first, because the lower or conversion block decides which your gun takes.
Can I use my AR-15 lower for 9mm?
Yes, with a 9mm conversion block or magwell for the magazines. A dedicated 9mm lower such as Aero's EPC-9 is built for Glock magazines and for blowback loads.
What buffer does an AR-9 need?
About 8 oz for a straight blowback gun (Aero's EPC buffer), more for heavier bolts. CMMG's radial delayed blowback guns do not need a heavy buffer.
Do I need a gas system on a 9mm AR?
No. Blowback and delayed-blowback AR-9s do not use a gas block or gas tube.
