https://sunnystateoutdoors.com/coll...ts/aim-h4-exo-bolt-carrier-group-5-56-223-mpi
9310 Bolt Material; 8620 carrier; Properly staked machined billet gas key hardened to USGI specifications fastened with Hardened Grade 8 Fasteners; Shot peened MPI (Magnetic Particle Inspected) Bolt. Extractor insert installed. Proudly made in the USA with a Lifetime Warranty.