American Built Arms Company Pro 5.56mm Bolt Assembly Silver/Black Small. For superior reliability these Bolt Carrier Groups feature a nickel boron coated bolt and extractor each crafted from heat-treated steel. A nickel boron coated bolt assembly has reduced friction and requires less maintenance than standard assemblies. Drop this AB Arms 5.56 bolt assembly into your AR-15 to guarantee your rifle will perform when it matters most. Specifications for American Built Arms Company Pro 5.56mm Bolt Assembly: Manufacturer: American Built Arms Company Color: Silver/Black Finish: Nickel-Boron Gun Model: AR-15 Features of AB Arms Company Mil-Spec 5.56 Bolt Assembly: Bolt: 9310 Steel Heat-treated per Mil-Spec Requirements Shot Peened per Mil-Spec Requirements MPI Tested Material Nickel Boron Coated Extractor: Aircraft Quality E4140 Steel Heat-treated per Mil-Spec Requirements Shot Peened per Mil-Spec Requirements Nickel Boron Coated Package Contents: American Built Arms Company Pro 556 Bolt Assembly - Nickel Boron Coated What firearm models does the manufacturer say American Built Arms Company Pro 5.56mm Bolt Assembly is compatible with? The manufacturer suggests that American Built Arms Company Pro 5.56mm Bolt Assembly is compatible with AR-15American Built Arms Company ABA5.56B: Pro 5.56mm Bolt Assembly Silver/Black Small