Yankee Hill Machine 9384: The Yankee Hill Machine 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 mounted by the use of two clamping screws. It is manufactured from high quality steel, finished with a mil. spec. phosphate coating. It is 0.95\" in length and has a bore diameter of .75\".