URX 4 The URX 4 is a continuous top-rail system that utilizes the Keymod mounting brackets developed by Vltor and Noveske. This rail system is built stronger and more stable than previous designs due to the absence of a removable bottom rail. The modular layout allows an end user to fully customize exactly where to place accessories without the use of specialized tools, small parts, or additional forces applied to the barrel. The URX 4 is currently available for 5.56 rifles in 8.5”, 10”, 10.75", 13”, and 14.5” lengths, and is equipped with Knight’s unique IBN mounting configuration. The idea was to create a more modular, and thus more serviceable, rail system that still maintains its strength and stiffness. Th IBN system torques the rail onto the weapon and allows the rail to act as the barrel nut. This design was developed to increase stabilization and provide a simpler solution that requires less specialized tools to install and maintain. A full array of modular accessories, iron sights, and panel kits utilizing the Keymod mounting brackets have been developed specifically to accompany this rail system. With the introduction of the URX 4, Knight’s Armament Co. demonstrates its continuing determination to intelligently design effective products. It is not suggested to use this rail with a 14.5" or shorter barrel unless using a muzzle device such as the AAC flash hider which can be installed from the end with a tool.