BCM Bolt Carrier Group (MPI) – Auto BCG

About this product

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The BCM Bolt Carrier Group (MPI) – Auto BCG delivers uncompromising reliability for serious builds, meeting USGI specifications with precision machining and tested durability.

What you get

• Complete Mil-Spec BCG for AR15/M16/M4 platforms
• Parkerized exterior with chrome-lined carrier interior
• Carpenter No. 158® steel bolt assembly (shot-peened)
• Chrome-lined, heat-treated gas key secured with Grade 8 fasteners
• Properly staked gas key for consistent gas system performance
• Tool steel extractor and ejector with BCM Extractor Spring
• HPT (High Pressure Tested) and MPI (Magnetic Particle Inspected)

Build and finish

The BCM BCG starts with a carrier machined from 8620 steel, parkerized for corrosion resistance and chrome-lined internally to reduce friction and carbon buildup. The bolt uses Carpenter No. 158® steel—the same alloy specified for military contracts—with shot peening to improve fatigue resistance. Critical surfaces like the cam pin track and gas rings are honed to tight tolerances.

Gas key staking follows the USGI double-stake method, preventing loosening under recoil. The extractor spring uses BCM’s proprietary design for consistent tension, while the chrome-lined key ensures smooth gas flow. Every component is individually inspected post-assembly, including MPI testing for structural flaws.

Compatible platforms

• AR-15 rifles and pistols (5.56/.223)
• M16-pattern firearms
• M4-pattern firearms
• Compatible with most Polymer80 frame builds using mil-spec upper receivers

Installation notes

This drop-in BCG requires no fitting for mil-spec AR-15 uppers. Apply a light coat of lubricant to the carrier rails and bolt lugs before initial use. Check gas key tightness after the first 200 rounds as part of standard break-in procedure.

For Polymer80 builders, this BCG pairs with mil-spec upper assemblies to create a reliable foundation for custom builds. The tested durability ensures consistent cycling whether used in compact pistol configurations or full-size rifle platforms.

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