CMMG Barrel, 300 AAC Blackout, 16.1 Barrel, 1:7 Twist, Carbine 4140 CrMo Salt Bath Nitride Finish, 5/8-24 TPI 30D120A Perfect for building your custom AR15 upper group. Comes with properly installed M4 barrel extension. All barrel sub-assemblies are properly head spaced.