The UTG Pro M4/AR-15 Carbine Length Drop-in Quad Rail with extension top rail extends to upper receiver to form continuous mounting platform.The top/bottom two-piece design has integral spinal cords/alignment housing for a perfect fit with the barrel nut and round front cap. It is precision CNC-machined from from aircraft aluminum with anodizing/engraving. The four Picatinny rails each have 14 slots with great zero-retention capability for all installed accessories. Easy installation with no modification to the barrel. Black rubber rail guards included. Note-This quad rail system is designed to fit with standard AR-15 gas tube. The compatibility with any special piston system is not guaranteed.