CMMG Barrel, 350 Legend, 16.1, 1:16 Twist, Carbine Length, Black SBN Finish, Threaded 1/2-28 35D5F0A Perfect for building your custom Mk4 upper group. Comes with properly installed M4 barrel extension. All barrel sub-assemblies are properly head spaced. Made of 4140CM with a SBN Finish.