The Advanced Technology International (ATI) Mil-Spec Buffer Tube Kit is a complete carbine receiver-extension assembly for AR-15 and AR-15 variant lowers. Machined from 6061-T6 aluminum and finished in military Type III hardcoat anodize, the extension is tough enough for high-round-count use without the wear or thread stripping you see on cheaper commercial-spec tubes. Specs and features: Mil-spec diameter (1.148" OD) - pairs with any mil-spec collapsible stock 6061-T6 aluminum, Type III hardcoat anodized Includes steel E-coated castle nut, receiver end plate, and locking ring Includes AR-15 carbine buffer and action spring Made in USA Compatibility: fits AR-15 and most AR-15-pattern (5.56 / .223 / .300 BLK / 9mm PCC) lower receivers. NOT compatible with .308 / AR-10 platforms - those require a large-frame buffer tube. Confirm your stock is mil-spec (not commercial-spec) before installing this tube; the two are not interchangeable.