AR15/ M16 Bolt Assembly, Nickel Boron Finish. Replaces your factory bolt head assembly and fits into your Bolt Carrier. Compatible with all AR15/M16 milspecs but made better with superior materials and production processes. Our bolts are made with SAE 9310 steel for longer life and greater reliability over the milspec C-158 steel. A special heat treating process comes next. Then each bolt is shot peened and plated with Nickel Boron for the ultimate in slick surfaces and reliable function. When to Replace Your Bolt Assembly: We recommend replacing the Bolt Assembly whenever you replace your barrel. An AR15 bolt head and barrel will have a certain amount of wear-in between them. So, it just makes sense to use a new bolt head with a new barrel.