ProMag PM138 AR-15/M16 Flip-Up Front Sight for Gas Block Mount, Black Front sight body and stem are machined from T6 aircraft grade aluminum and type III hard anodized Precision machined to fit dovetailed gas blocks meeting MIL-STD 1913 Picatinny rail specifications The sight is mounted to the gas block via a locking bar and secured with a cross slotted, knurled thumb screw No permanent modification required for installation The PM138 sight may be paired with model PM136, PM137, PM200 PM202 rear sights mounted to AR-15 / M16 flat top A3 upper receiver rifles and carbines