CMMG BBL 300BLK 16.1″ MR 4140CM SBN

Description

This barrel sub-assembly is built for shooters putting together a dedicated .300 Blackout upper — whether that means a compact suppressor host, a subsonic-capable hunting build, or a general-purpose carbine that needs to cycle both supersonic and subsonic loads without swapping parts. CMMG machines the barrel from 4140 chrome-moly steel and applies a Salt Bath Nitrided finish, which hardens the bore and exterior surfaces to resist corrosion and reduce friction without requiring a chrome-lined bore.

The carbine-length gas system is a meaningful choice for a 16-inch .300 Blackout barrel. It taps gas closer to the chamber than a mid-length setup would, which produces a sharper but shorter dwell-time impulse. On a direct-impingement rifle, that translates to a snappier but more controllable recoil character, and it keeps the action cycling reliably across the wide range of pressures that subsonic and supersonic .300 Blackout loads generate. The 0.750-inch gas block diameter is a standard journal size, so fitting a low-profile or adjustable gas block is straightforward.

The 1:7-inch twist rate is optimized for heavier projectiles — the 208- to 220-grain subsonic bullets commonly loaded in .300 Blackout stabilize well at this rate, and it handles standard 110- to 125-grain supersonic loads without issue. The muzzle is cut to 5/8×24, the standard thread pitch for .300 Blackout, which keeps suppressor and muzzle device selection wide open. The barrel ships with a properly installed M4 barrel extension that has been headspaced at the factory, so it drops into a standard AR-15 upper receiver ready for a bolt carrier group.