The UTG Pro Ops Ready S4 is a complete mil-spec AR-15 stock kit - buttstock, receiver extension, buffer, spring, castle nut, and end plate in one drop-in package. Molded from impact-resistant matte black polymer, the S4 buttstock is compact at 4.28" and offers 4-position length-of-pull adjustment, which is enough range to accommodate a plate carrier or heavy jacket without losing a solid cheek weld. Specs and features: Complete mil-spec AR-15 stock kit - nothing else to buy Includes UTG Pro S4 buttstock, mil-spec 6061-T6 aluminum receiver extension, carbine buffer, action spring, receiver end plate, and steel castle nut 4-position length-of-pull adjustment Compact 4.28" stock body - good for shorter LOP shooters and armored setups Matte black impact-resistant polymer Tight tolerance fit on mil-spec extensions (will not rattle on the tube) Compatibility: fits any AR-15 lower running a mil-spec (not commercial) diameter carbine buffer tube. Because this kit includes the buffer tube, it is a full replacement - you do not need an existing tube. Not for A2 fixed-stock rifles or AR-10 platforms.