This gas block replaces your existing front sight and allows the user to mount accessories to its top picatinny rail while retaining the bayonet lug and sling mount. It is mounted by the use of four socket head cap screws for easy installation.This gas block allows the mounting of YHM forearms without the removal of your muzzle accessory. Purchase YHM's dovetail mounted flip front sight (YHM-9584) or our solid front sight (YHM-9495) to retain your iron sights. Made from high quality steel and finished with a Mil. Spec. phosphate coating.