Picatinny Aluminum Quad Rail For Ar15/M4 Upper three rail section mount underneath the end cap and secured in place with a friction fit ledge and roll pins. The receiver end secured with a barrel nut bracket that is atached to the upper three rail section with four captive flathead screws. The bottom rail is secure with friction fit ledge and with a retractable spring loaded piston. Four full length rails – top rail creates one long monolithic rail with the existing flat top rail. XM4S has two push button sling swivel mounting points at the front. XM4SD has four push button sling mounting points at the front and back. Slots around the rails provide maximum ventilation around the rails. CNC machined from aviation grade aluminum to MIL-1913 standards with a hard anodized black oxide finish for durability and ease of maintenance. Accepts grenade-launcher.