M4-style receiver extension tube has six precision-machined notches on the underside for maximum flexibility in setting length-of-pull with adjustable buttstocks. Lets you install a mil-spec stock on a civilian carbine or refit a rifle with an A2-style fixed buttstock with an adjustable M4 stock. Outside diameter of tube is machined approximately .003" oversized for snug, rattle-free fit of stocks designed for mil-spec extensions. Lightly sand interior of stock to get just the right combination of snugness and freedom over movement. Machined from a lightweight aluminum extrusion before hardcoat anodizing with a wear-resistant matte black finish. Rifle conversion also requires a carbine action spring and buffer, available separately.