The Yankee Hill Rifle Forearm Tubes are a great addition to any AR-15. They feature slotted heat vents, mounting point for Bipod Stud (YHM-262) and they also accept Forearm End Caps (YHM-9484A or YHM-9484B sold separately). Each tube is drilled and tapped at 45 degree angles to accept custom rails at 8 different positions. Extra rails can be purchased separately. All assemblies include: one jam nut, one barrel nut, one full length rail, one elevated 3 inch rail, and one forearm tube. Accepts 3, 4, 6, 7, 9, 11, and 12 inch rails. Includes a 12 inch picatinny rail and a 3 inch elevated rail!