The Yankee Hill Machine YHM-9381A Low Profile 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. The gas block is mounted by the use of two set screws. The Low Profile Gas block is manufactured from high quality steel and finished with a mil. spec. phosphate coating. This gas block has a bore diameter of .750 inches. Sturdy and durable.