The Knight's Armament URX 4 M-LOK Rail is a one-piece, fully modular system. With its Integral Barrel Nut system, the URX 4 is a stronger, lighter improvement over previous URX designs. The IBN system allows the rail itself to act as the barrel nut when torqued onto the rifle, eliminating the need for a separate steel barrel nut. M-LOK accessories may be mounted anywhere along the URX 4's length, making it one of the most user configurable designs on the market.Features: Installation wrench, shim kit and instructions. Length: 10" Inside Diameter: 1.55" Outside Diameter: 1.68" (Side to Side) Weight: 9.2 oz