BCM® (MPI) complete bolt carrier group is fully assembled. (BCG) Bolt carrier group (auto version) for your AR15, M16, or M4. Parkerized exterior and chrome lined inside carrier. Machined to USGI specifications. This M16/M4 carrier includes the proper Mil-Spec gas key. Gas key is chrome lined and heat treated per GI specifications. Gas key is secured to carrier via USGI domestic mfg Grade 8 fasteners and properly staked per Mil-Specs. These properly hardened parts insure a strong staking and proper gas system functioning. The bolt assembly is machined from the correct Mil-Spec Carpenter No. 158® steel, shot peened for increased strength, includes tool steel machined extractor and ejector, BCM Extractor Spring, and is HPT (High Pressure Tested) and MPI (Magnetic Particle Inspected). Prior to shipping, all BCM Bolt Carrier Groups are test fired for function. As such, the Bolt Carrier Group will show handling marks consistent with these quality control procedures. The Flat Dark Earth version has the stripped carrier finished with an Ion Bond coating which lowers friction and allows for easier cleaning.
Big Book Catalog, Issue:73, Page:011
Big Book Catalog, Issue:73, Page:017