Clever Top/Bottom Two-Piece Design with Integral Spinal Cords/Alignment Housing for Great Fit * Designed with Innovative Alignment Tabs at the Front for Perfect Fit with the Standard Triangular Front Cap, Fits Mid. Length Model 4/15, Includes Four Picatinny Rails Each with 18 Slots * Great Zero-Retention Capability for All Accessories Installed on the Rails, Easy Installation with No Modification to the Barrel, Black Rubber Rail Guards Included * Proudly Made in USA with Superb Design and Production * This quad rail system is designed to fit with standard AR15 gas tube. The compatibility with any special piston system is not guaranteed.