Aero Precision Bolt Carrier Group, 9mm, Fits AR9, Nitride Finish, Black APRH200083C The Aero Precision EPC/AR9 Bolt is the core of your EPC build. Machined from 8620 steel and featuring a nitride finish" fracture resistant grade 23 titanium pin" and 4140 steel extractor" the EPC bolt assembly is precision machined from the ground up to meet the extreme demands of blowback AR platform firearms.