Faxon Firearms Duty Series, 1:8 Twist Barrel, 556NATO, 16 Pencil Profile, Mid-Length Gas System, Fits AR-15, Nitride Finish, Black 15A58M16NPQ Manufactured in-house" these barrels are fully stress relieved" air-gauge tested" and 11 degree target crowned to ensure superior accuracy. Quality is imbued into their barrels from the start. They start with the best materials" either certified 4150 Mil-Spec steel or 416-R Stainless. From there" each barrel goes through a battery of tests at each operation until finished in Nitride and final inspected. All barrels are individually checked for headspace by precision custom hardened gauges.