Knight's Armament FF RAS (SR-25 MK11 MOD 0). Rail is approximately 12 1/4" long including the barrel nut. Free floats the barrel. Picatinny rails on all four sides allow endless possible mounting solutions for accessories such as sling mounts, vertical forearm grips, flashlights and more. The RAS comes with three 11 rib panels to help protect the rails from damage. Will not fit the Armalite AR-10. Installation available, special tools required.