The Yankee Hill 4 Rail Gas Block replaces your existing front sight and allows the you to mount accessories to its four picatinny rails. This gas block can be mounted either by drilling and pinning it in place or by the use of set screws (included). The 4 Rail Gas Block is made from high quality steel and finished with a Mil. Spec. phosphate coating for years of use. This gas block has a bore diameter of .750 inches.