BRN-4 Complete Bolt Carrier Group for 416 Type Firearms This Completely assembled BRN-4 Bolt Carrier Group is ready to drop into HKA(R) 416, MR556 or other compatible upper receivers & firearms. Utilizing high quality materials, finishes and inspection criteria, this BCG provides great value as a replacement or build component. The Brownells BRN-4 bolt is machined from high performance 9310 steel, heat treated, magnetic particle inspected for defects and finished in Black Nitride. The Black Nitride finish combined with the piston operating system makes for extremely simple cleanup. BRN-4 Bolt Carrier Group is designed to be compatible with standard AR-15 o Mfg: Brownells