The Guide to AR-15 Receiver Compatibility

We recently covered how to pick the right magazines for your AR-15’s setup. But there are still plenty of questions about cartridge compatibility and types of magazines. Let’s clear up some of the confusion: Here’s all the answers to your questions about magazine compatibility with the AR’s most popular rounds.

AR Magazines are Split Into Two Categories

 Those two categories are based on the AR platform itself, which comes in two flavors: There’s the original AR-15, with short-action receivers that chamber rounds like 5.56 NATO, 300 Blackout, and 6.5 Grendel, and there are long-action receivers that make up the AR-10 – which chambers the .308 Winchester, and other, larger centerfire rifle cartridges.

We’ll also cover some edge cases, like rimfire cartridges and pistol-caliber ARs.

Which Calibers fit in the AR-15’s Standard Magazine?

This list is long, with some uncommon and wildcat rounds filling the field. The most popular short-action cartridges that fit in the AR-15’s 5.56/.223 magazine with a double-stacked configuration include:

  • .17 Fireball
  • .17 Remington
  • .19 Calhoun 
  • .20 Tactical 
  • .204 Ruger
  • .224 Valkyrie*
  • .25-223
  • .300-221 Fireball 
  • .300 AAC Blackout
  • .375 Reaper
  • 7mm Whisper
  • 7.62 X 40mm 
  • 6 X 45mm 
  • .257X45mm
  • 6.8 SPC*

The .223 Valkyrie and 6.8 SPC cartridges can only be loaded to partial capacity due to magazines bulging when loaded to higher capacities. Although you can load these rounds in 5- to 10-round 5.56 magazines, it’s recommended to instead purchase dedicated magazines for these rounds.

These rounds fit in the AR-15’s magazine in a single-stack configuration:

  • .440 Corbon
  • .458 SOCOM 
  • .458 RMW
  • .50 AE
  • .50 GI
  • .50 Beowulf

Which Calibers fit in the AR-10’s Standard Magazine?

The larger action of the AR-10 was designed to chamber high-power rifle cartridges developed for long-range shooting, precision target shooting, and hunting. The below cartridges fit in the AR-10’s .308 Winchester/7.62×51 NATO magazine:

  • 6.5 Creedmoor
  • .22-250 Remington
  • .243 Winchester
  • .260 Remington
  • 7mm-08 Mauser
  • .338 Federal 
  • .358 Winchester
  • .450 Marlin

What About Rimfire and Pistol Cartridges, and Custom ARs?

Rounds like .22 LR, 9mm, and .45 ACP require their own, dedicated magazines if they’re to be chambered in the AR platform. While all these rounds fit in the AR-15’s receivers, they require something called a “conversion block” – a metal insert that secures inside the magazine well – to allow the magazine to interface with the upper receiver, bolt, and barrel extension.

ARs chambering pistol and rimfire rounds also come prebuilt to accept their respective magazines. The AR9, for example, is designed to use GLOCK and COLT SMG magazines. ARs chambered in .22 LR use a proprietary magazine designed to handle those tiny casings.

But custom-built ARs that chamber centerfire rifle cartridges – including those build using 80% lowers, whether for the AR-15 or AR-10 – adhere to “mil-spec” standards. That means they, too, use standard magazines for their respective receivers, and the same rules apply when it comes to magazine compatibility with the calibers above.