Knight's Armament Free Float Rail system with rails that extend around and beyond the front sight base, hence the MRE ("More Real Estate") designation.
This rail set-up was developed by KAC at the request of the Spec Ops community, to give the operator the same rail area as a 20" rifle.
Although this rail was designed initially for the Carbine gas system, the front filler tube has been milled to be used with a Rifle gas system, for maximum operational flexibility.
For more info check out this link:
$350 shipped. No trades please.