RISE Armament AR15 Bolt Carrier Group - Nickel Boron Finish - When it comes to your AR-15's bolt carrier group, reliability is key; that's what you get with RISE Armament's advanced Nickel Boron Bolt-Carrier Group (BCG): reliability. So why is nickel boron so great? For that, here's a little science. Nickel boron (NiB) is a columnar-formed compound. It's about 95 percent nickel and 5 percent boron. Due to its formation, it reduces contact points between surfaces. However, it still provides an additional layer of hardness to the steel. Because of the reduced surface contact-given only the rounded ends of the NiB column's touch-it acts almost like a lubricant. Features: Unit weight: 11.6 oz. Individually MPI tested Bolt: 9310 MPI tested Carrier: 8620 tool steel heat treated and case hardened Gas key: tool steel - grade 8 hardened fasteners and staked properly Cam pin: 4140 steel Extractor: tool steel - shot peened