The CMMG 12.5-inch 300 Blackout barrel is a headspaced AR-15 barrel sub-assembly from CMMG (CMMG part no. 30D8A0A), sold with the M4 barrel extension already installed. CMMG machines it from 4140 chromoly steel in its Multi Role contour, rifles it with a 1:7 twist for heavy 300 Blackout bullets, and finishes it in salt bath nitride. The gas port sits at the pistol-length position and the journal takes a .750-inch gas block. It is made for building a custom AR-15 upper group without having to install the extension or set headspace yourself.

Key Specifications

• The caliber is .300 Blackout, the .30-caliber AR-15 round built around standard bolts and magazines.
• Barrel length is 12.5 inches, a compact length that keeps the upper short and handy.
• The twist rate is 1:7, suited to the heavy bullets common in subsonic 300 Blackout loads.
• The contour is Multi Role, CMMG’s general-purpose profile.
• The barrel material is 4140 chromoly steel, a tough alloy widely used for AR-15 barrels.
• The finish is salt bath nitride, which hardens the surface and resists corrosion inside and out.
• Muzzle threads are 5/8×24, the standard pattern for .30-caliber muzzle devices.
• The gas port is at the pistol-length position, matched to the short barrel and cartridge.
• The gas block journal is .750 inch, the common size for low-profile gas blocks.
• The barrel weighs 2 pounds and comes with the M4 barrel extension installed and headspaced.

Frequently Asked Questions

Is the barrel extension installed? Yes. CMMG ships this sub-assembly with a properly installed M4 barrel extension, and every barrel sub-assembly is headspaced.

What gas tube does it need? The gas port is at the pistol-length position, so it takes a pistol-length gas tube with a .750-inch gas block.

What does the Multi Role contour mean? It is CMMG’s general-purpose barrel profile, balanced between weight and rigidity for a do-everything upper.

Buy the CMMG 12.5-Inch 300 Blackout Barrel, Multi Role Contour, Salt Bath Nitride

Buying the barrel as a headspaced sub-assembly saves a builder the most exacting step of assembling an upper. Add it to your project from our AR-15 barrels and we will ship it as soon as it is ready from our Lubbock, Texas gun shop.