The KAC URX III & 3.1 Rail 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. Available in 8"or 13.5" lengths. 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 install Mfg: Knights Armament SPECS: Fits - AR15/M16 rifles. Free Float. Material: Aluminum. Finish: Black Anodizing. 8" Weight - 8.6 oz. 13 .5 - Weight - 13.8oz.