CMMG BOLT CARRIER GROUP MK3 308

Description

CMMG’s Mk3 bolt carrier group is a complete, drop-in replacement designed for AR-platform rifles chambered in .308 Winchester and built around the standard large-frame AR-10 pattern. It suits builders finishing a new upper or shooters replacing a worn or mil-spec group who want a known-quantity part from a dedicated AR manufacturer rather than a generic substitute.

The carrier and bolt are machined from steel and finished in black phosphate, a treatment that provides corrosion resistance and retains lubrication well across a range of operating conditions—useful in a semi-automatic that runs a high volume of brass through a gas-impingement or adjustable-gas system. The carrier interior is chrome lined, which reduces friction at the point of highest heat and wear and makes cleaning noticeably easier after extended range sessions. Chrome lining does not change the exterior dimensions of the carrier, so fit in a standard Mk3-spec upper remains consistent.

Because this is sold as a complete group, it arrives assembled and headspaced-ready to install—the bolt, extractor, ejector, cam pin, firing pin, and retaining pin are all included. That matters when you are building from scratch and want to avoid sourcing individual components that may or may not clock correctly together. CMMG’s Mk3 designation indicates the group is spec’d for their large-frame .308 platform, so verify your upper receiver’s bolt carrier group pattern before ordering if you are working with a third-party upper.