FN15 16 Carbine Upper (HF Barrel) FN 15 Upper Receiver Assemblies are meticulously built in FN's Columbia South Carolina manufacturing facility alongside US Military contract M4 rifles. Each upper receiver is supplied with and M16-style Bolt Carrier Assembly with properly staked gas key and Carpenter 158 Magnetic Particle Inspected (MPI) bolt, charging handle and FN's venerable cold hammer-forged barrel. Specifications Caliber 5.56 NATO Upper Receiver Flattop Forged, M4 Feedramps Barrel Length 16" Barrel Profile Government Barrel Material (HF)Hammer Forged 4150 CMV Muzzle 1/2 x 28 , A2 Flash Hider Twist 1:7" Gas System Carbine Length Direct Gas Bolt Carrier Group M16, Carpenter 158 Bolt(MPI) Handguard Drop In Carbine Charging Handle Mil-Spec