These rails attach to the accessory slot at the front of the Comp Chassis and allow you to attach forward grips or bipods. They come in two sizes – 3-slot and 7-slot. The 3-slot rail has 3-1/2 M-LOK slots on the bottom and the 7-slot has 7 slots. Total length of the 3-slot rail is 6.5″ and it only weighs 1.8 ounces. the 7-slot rail is 12.25″ and only weighs 2.9 oz. The rails are CNC-machined from 6061T6 aluminum, so they are very tough and rigid. The rail has a QD sling swivel hole with anti-rotation stops at the end.