Knight's Armament FF RAS (SR-25 Carbine). Rail is approximately 10" long. 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. 
 Will only fit SR25 or DPMS pattern uppers, will not fit the Armalite AR-10. Installation available, special tools required.