Aim Sports MT021: This M4 carbine-length, drop-in quad rail by Aim Sports is an easy to install upgrade for the basic carbine. Made from aircraft-grade aluminum, it is compatible with most carbine-length direct impingement gas systems. The two-piece design requires no gunsmithing to install and is compatible with all standard Weaver/Picatinny 1913 accessories. It is 6.5\" long and weighs 11.03 oz. This model has a black anodized finish.