This gas block replaces your existing front sight to provide a simple low profile design that can be used to mount a rifle length forearm to a carbine length barrel. It is Hinged to go around the barrel with a pinned and welded muzzle device. The two screws take a 9/64 Allen Wrench – Specifications – Low Profile – .750 Bore Diameter – Clamp Screw Mounted – Weight: 1.98 Oz