CMMG BBL 300BLK 16.1″ MT 4140CM SBN

Description

This barrel sub-assembly is built for shooters putting together a carbine-length .300 AAC Blackout upper — a configuration that works well for hunting medium game, suppressed shooting, or general-purpose range use where a standard-length rifle makes sense. The sub-assembly ships with a properly installed M4 barrel extension, and CMMG confirms headspacing is verified at the factory, so you are not inheriting alignment problems to sort out during your build.

The 4140 chrome-moly steel construction handles the pressure demands of .300 BLK across both supersonic and subsonic loads without the extra weight that comes with heavier contour options. The MT contour strikes a balance between keeping the assembly light and providing enough mass to manage heat during sustained fire. The Salt Bath Nitride finish — SBN — penetrates the steel surface rather than sitting on top of it, which means the bore and exterior surfaces both benefit from the same corrosion resistance and reduced friction, an advantage over finishes applied only to the exterior.

A 1:7-inch twist rate stabilizes the heavier subsonic projectiles that define much of .300 BLK’s practical appeal — bullets in the 200–220 grain range that would not stabilize reliably in a slower twist. It also handles standard supersonic loads without issue. The carbine-length gas system produces a sharper but shorter recoil impulse compared to a mid-length setup; in a direct-impingement build, that is worth factoring into buffer weight selection. The 5/8×24 muzzle thread accommodates the suppressors and muzzle devices most commonly paired with this caliber. The 0.750-inch gas block journal fits the most widely available aftermarket gas blocks.