CMMG 300 Blackout 16.10" Barrel Sub-Assembly, Black Nitride, 4140 Chromoly Steel, Carbine Length, Medium Taper, AR-15

Building a reliable .300 Blackout AR upper doesn't have to mean guessing on parts. The CMMG 300 Blackout 16.10" barrel sub-assembly gives you a complete, headspaced foundation you can trust—no trial and error. This is the stuff precision shooters and serious builders depend on: a 4140 chromoly steel barrel finished in hard black nitride, paired with a properly installed M4 extension, ready to feed into your lower. Whether you're assembling a dedicated subsonic rifle for hunting or a modular platform for the range, a quality barrel sub-assembly cuts the guesswork out of upper assembly and gets you shooting faster.

The 16.10-inch medium-taper profile strikes a practical balance—light enough to handle all day, stiff enough to shoot straight. Carbine-length gas system tuning keeps recoil manageable and cycling reliable across the broad spectrum of .300 Blackout loads. The 1:7 twist is built to stabilize the heavy, long bullets that make this cartridge sing, whether subsonic hunting rounds or faster loads for medium game. Those 5/8-24 muzzle threads open up your options for suppressors, brakes, or flash hiders.

Black nitride finish delivers serious corrosion resistance without the bulk or cost of stainless, and it's proven in hard use. The fact that this sub-assembly arrives headspaced and ready means you're not waiting on a gunsmith or wondering if something's out of spec—you can focus on the rest of your build and get to the range with confidence.

Key Features

• Pre-installed M4 barrel extension—no additional headspacing needed
• 4140 chromoly steel barrel with black nitride finish for durability and corrosion resistance
• 16.10-inch medium-taper contour for balance between weight and accuracy
• 1:7 twist rate optimized for .300 Blackout heavy-bullet performance
• Carbine-length gas system for reliable cycling
• 5/8-24 muzzle threads for suppressors and accessories
• Complete sub-assembly ready to install—no additional headspacing required

Specifications

Frequently Asked Questions

Is this barrel sub-assembly ready to install?

Yes. The M4 barrel extension is pre-installed and the assembly is headspaced to specification, so you can mount it directly to your upper receiver.

What .300 Blackout loads will this 1:7 twist stabilize?

The 1:7 twist is built to stabilize heavy .300 Blackout rounds, including the subsonic loads that define this cartridge's performance envelope.

Can I use a suppressor with the 5/8-24 threads?

Yes. The 5/8-24 muzzle threads accept .300 Blackout suppressors and other threaded accessories that use that standard.

What's the difference between black nitride and stainless steel?

Black nitride offers excellent corrosion resistance and durability in a harder finish than stainless, with less weight and typically lower cost. It's a proven choice for AR barrels in hard use.