These tubes are a great addition to any AR-15. They feature slotted heat vents, mounting point for a Bipod Stud (100-005-267) and they accept a Forearm End Caps (100-002-086 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. Accepts 3, 4, 6 , and 7 inch rails. (100-002-081, 100-002-082, 100-003-157, 100-002-083) Barrel nut is not included. These are surplus parts and as such may have scuffs, scratches or other defects.