YHM’s 4 Rail Forearms features include: mounting points for YHM’s Swiveling Sling Studs (YH9469 sold separately), Mil. Spec. Picatinny rails and they accept YHM’s Forearm End Cap (YH9484 sold separately). The barrel nut and forearm tube are made of aircraft quality aluminum and hard coat anodized to Mil. Specs.. The jam nut is made from quality steel and phosphate coated. All assemblies include: one jam nut, one barrel nut, and one forearm tube.