Convenient kit contains all the components needed to mount the buttstock of your choice on an AR-15 rifle or carbine. Perfect for stock conversions such as altering a rifle to accept a collapsible carbine stock. All parts are manufactured from the highest quality materials and held to strict dimensional tolerances for proper fit and function. Commercial carbine kit comes with 1.168" O.D., six-position receiver extension for use with commercial or "universal" buttstocks. Mil-Spec kit has a 1.148" O.D., six-position receiver extension for correct fit of military M4 carbine buttstocks. M4 buttstocks and clones will not fit Commercial tubes. Mfg: Brownells SPECS: Receiver Extension & A2 Stock Spacer - Aluminum, anodized, matte black. Recoil Buffer - Aluminum and steel. Spring, Lock Ring, Receiver Plate - Steel. Kits fit standard, mil-spec AR-15/M16/M4 lower receiver and clones. Installation instructions included.