Knights Armament Co.'s URX 3.1 13.5" Rail Forend Assembly is a free floating" fully modular replacement forend for the AR-15 platform. Unlike traditional quad rail designs" the URX 3.1 incorporates short sections of M1913 Picatinny rail at its front and rear ends for mounting tactical accessories" eliminating the commonly unused sections of rail for reduced overall weight and a more comfortable grip. A total of 4 integral QD sling swivel sockets are provided for ambidextrous sling compatibility. This URX 3.1 rail is compatible with any standard gas system length but does require the use of a low profile gas block. Disassembly of the rifle's gas system and removal of the factory barrel nut will be necessary. This forend uses a proprietary barrel nut that MUST be installed using a KAC URX Barrel Nut Wrench (PN: 20495" Sold Separately). For AR-15/SR-15 based platforms only; not compatible with SR-25 variants. Installation instructions" barrel nut and hardware are included.Free floating 13.5" rail forend assemblyFully modular replacement forend for the AR-15 platform