Caliber Conversion: Building the AR in Non-Standard Calibers

The AR platform’s modularity makes caliber conversion one of its most appealing features—but it’s not as simple as swapping barrels. Each AR class—pistol caliber, small frame, and large frame—has its own standards, and converting between calibers within a class often requires changes to multiple components. From bolt carrier groups and barrels to magazines and buffers, this guide breaks down exactly what you need to modify when changing calibers within your chosen AR platform. Whether you’re moving from 5.56 to .300 Blackout or .308 to 6.5 Creedmoor, we’ll help you identify what stays, what goes, and what needs fine-tuning.

Caliber-Specific Component Compatibility

EPC Calibers

Caliber Upper Receiver Muzzle Device Barrel Bolt Magazine
9mm Luger Standard Standard Standard Standard Standard
40 S&W Standard .40 Cal .40 S&W .40 S&W .40 S&W
Pistol
Carbine
Pistol
Carbine

AR-15 Calibers

If you are interested in learning about the AR-15 cartridges compatible with the AR-15 platform, check out THIS ARTICLE by GunDigest.
Caliber Upper Receiver Muzzle Device Barrel Gas System Bolt Magazine
5.56x45mm Standard Standard Standard Standard Standard Standard
.223 Remington Standard Standard Standard Standard Standard Standard

.223 Wylde1

 Standard Standard .223 Wylde Standard Standard Standard
.204 Ruger Standard Standard .204 Ruger Standard Standard Standard
.22 LR Standard Standard Standard

N/A2

 .22 LR BCG .22 LR
.22 Nosler Standard Standard .22 Nosler Standard Standard Standard
.224 Valkyrie Standard Standard .224 Valkyrie Standard 6.8 SPC 6.8 SPC
6.5 Grendel Standard .264 Cal 6.5 Grendel Standard 6.5 Grendel 6.5 Grendel
6.8 SPC Standard .30 Cal 6.8 SPC Standard 6.8 SPC 6.8 SPC
.300 Blackout Standard .30 Cal .300 BLK Standard Standard STD or .300 BLK
.300 Ham'r Standard .30 Cal .300 Ham'r Standard Standard .300 BLK
7.62x39mm Standard .30 Cal 7.62x39mm Standard

7.62x39mm3

 7.62x39mm
.458 SOCOM XL/Modified .458 Cal .458 SOCOM Straight Tube .458 SOCOM .458 SOCOM
  1. .223 Wylde is not a cartridge, but a chamber. It offers compatibility with both .223 Remington and 5.56x45mm cartridges, but with improved accuracy over a 5.56x45mm chamber.
  2. The .22 LR BCG is a direct blowback design and does not use the gas system for cycling. If your gun is equipped with an adjustable gas block, we recommend closing the gas system to prevent unnecessary fouling from the rimfire ammunition.
  3. In addition to a specific 7.62x39mm bolt, you will also need an enhanced firing pin to ensure reliable ignition of the 7.62×39’s hard primers.
Pistol
Carbine
Rifle

AR-10 Calibers

Caliber Upper Receiver Muzzle Device Barrel Gas System Bolt Magazine
.308 Winchester Standard Standard Standard Standard Standard Standard
6.5mm Creedmoor Standard .264 Cal 6.5mm CM Standard Standard Standard
.243 Winchester Standard .264 Cal .243 Win Standard Standard Standard
.260 Remington Standard .264 Cal .260 Rem Standard Standard Standard
Carbine
Rifle
*** This requires a 15.5″ gas tube. A standard 15.1″ (rifle length) gas tube will not work with this barrel.
*** This requires a 15.5″ gas tube. A standard 15.1″ (rifle length) gas tube will not work with this barrel.

Frequently Asked Questions

An AR-15 caliber conversion allows you to change the caliber your rifle fires by swapping components like the barrel, bolt, magazine, and sometimes the upper receiver. This flexibility lets you use your AR-15 as a multi-caliber platform without buying a new lower receiver.

To convert your AR-15 to 9mm, you’ll typically need a dedicated 9mm upper, a 9mm bolt carrier group (BCG), a compatible buffer system, and either a magwell adapter or dedicated pistol caliber lower. You’ll also need Glock- or Colt-style magazines depending on the setup.

Yes. The .300 BLK caliber was designed to work with standard AR-15 bolts and magazines. All you need is a .300 Blackout barrel and appropriate muzzle device. No changes to the bolt or mags are necessary, making this one of the easiest caliber conversions.

The best kits depend on your intended use. For 9mm, CMMG and PSA make reliable options. For .22 LR training, look at CMMG Bravo or Bore Buddy. For long-range (.224 Valkyrie or 6.5 Grendel), consider complete upper receivers with matched bolts.

Costs range from $150 to $800+ depending on the caliber. A .22 LR conversion bolt can be under $200, while a full upper in .458 SOCOM with specialized mags might cost over $700. Choosing a modular AR-15 lowers long-term conversion costs.

Yes, it’s legal under federal law as long as the resulting configuration complies with NFA and state laws (e.g., barrel length, overall length, and SBR definitions). Always verify local laws before converting.

Multi-caliber AR-15 setups can be extremely reliable when built with quality parts and tuned for each caliber. Proper buffer weights, gas port sizes, and magazines are critical for reliable function in alternate calibers.

Compatibility depends on your lower receiver type:

  • Pistol caliber lowers (e.g., Glock-mag lowers) work for 9mm, .40 S&W, etc.
  • Small frame lowers handle 5.56 NATO, .300 BLK, .224 Valkyrie, etc.
  • Large frame lowers support .308 Win, 6.5 Creedmoor, etc.

For compatibility of components across platforms, use our Component Compatibility guide.

Sometimes. .300 BLK uses standard 5.56 mags. Other conversions—like 9mm, 6.5 Grendel, or .458 SOCOM—require dedicated magazines. Always confirm magazine compatibility before converting.

  • Cost-effective: Reuse your lower and accessories.
  • Training efficiency: Use cheaper calibers like .22 LR for practice.
  • Mission flexibility: Switch between home defense, hunting, and range use with one rifle.
  • Space-saving: Fewer serialized receivers to store or transport.
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