CMMG 30D810A 8" 300 Blackout Barrel Sub-Assembly, Black Nitride, 4140 Chromoly Steel, Medium Taper, Pistol Length for AR-15

The CMMG 30D810A 8" 300 Blackout barrel sub-assembly converts your AR-15 platform into a formidable short-range and suppressed-fire machine without a complete rifle rebuild. This pistol-length gas system barrel gives you the ballistic authority of 300 AAC Blackout in a compact 8-inch package — ideal for short-barreled rifles, pistols, or any shooter who values maneuverability and modularity. Because 300 Blackout uses the standard AR-15 bolt carrier and magazines at full capacity, a barrel swap is all you need; no additional modifications required.

The barrel's 1-7" twist rate handles the full spectrum of 300 Blackout ammunition, from lightweight 110-grain supersonic rounds to heavy 220-grain subsonic loads perfect for suppressed shooting. The 5/8"-24 muzzle threads accept standard AR-.308 flash hiders and muzzle brakes, giving you flexibility in how you configure the muzzle. Forged from 4140 chromoly steel and treated with CMMG's proprietary Wasp nitriding process, this barrel resists wear and corrosion in demanding conditions — whether you're running it hard at the range or in the field.

The medium taper profile and pistol-length gas system positioning make this barrel a natural fit for short-barreled AR builds where weight and balance matter. If you already own an AR-15 platform, this barrel sub-assembly is a straightforward path to experimenting with 300 Blackout's unique blend of stopping power, subsonic capability, and shooter control.

Key Features

• 8" barrel length optimized for pistol-length gas systems
• 1-7" rifling twist compatible with 110–220 grain ammunition
• 4140 chromoly steel with black nitride (Wasp) corrosion and wear resistance
• Medium taper profile for balanced handling
• 5/8"-24 muzzle threads for AR-.308 suppressors and muzzle devices
• Drop-in replacement for standard AR-15 barrels; no bolt or magazine modifications needed

Specifications

Frequently Asked Questions

Will this barrel work with my existing AR-15 bolt and magazines?

Yes. The CMMG 300 Blackout barrel uses the standard AR-15 bolt carrier and accepts standard AR-15 magazines at full capacity. No additional parts or modifications are required beyond swapping the barrel.

What gas system does this 8" barrel require?

This barrel is designed for pistol-length gas systems. If you're building a short-barreled rifle or AR pistol, ensure your gas tube and receiver support pistol-length operation.

Can I use both subsonic and supersonic ammunition in this barrel?

Yes. The 1-7" twist rate accommodates the full range of 300 Blackout loads, from 110-grain supersonic to 220-grain subsonic rounds, making it versatile for both suppressed and unsuppressed shooting.

What muzzle devices are compatible with the 5/8"-24 threads?

The barrel accepts standard AR-.308 flash hiders, suppressors, and muzzle brakes with 5/8"-24 threads. This is a common thread pattern for short-barreled AR platforms and gives you many options for customization.