These rails attach to the accessory slot at the front of the Comp Chassis and allow you to attach forward grips or bipods. The bottom of the rail has 3-1/2 M-LOK slots. Total length of the rail is 6.5″ and it only weighs 1.8 ounces, so it is a great addition to the comp chassis for a lightweight charger or carbine build. 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.