This Guntec BCG was made for 7.62x51 / .308 Cal Platforms.  It is Nitride treated, HPT and MPI tested and marked with a 9310 Hardened Steel Bolt, 17-4 Stainless Steel Firing Pin (Heat treated to H900 Condition), and an 8620 carrier and gas key.  Made in the USA, this group comes fully assembled with properly torqued and staked gas key. Back Specifications Bolt Finish: Salt Bath Nitride Bolt Material: 9310 Steel Brand: Guntec USA Caliber Gauge: .308 Win Carrier Finish: Salt Bath Nitride Carrier Material: 8620 Carpenter Steel Carrier Style: DPMS Pattern Carrier