The revolutionary patented FAB Defense 'GL-Shock' absorbing Buttstock contains a recoil-reducing mechanism with a built in premium silicon chrome alloy spring creating a State-of-the-Art stock for your M4/M16. FeaturesUnique Patented "Anti-Rattle" MechanismSleek Design with No Protruding Parts,Will Not Get Caught on Gear and WebbingUnique Slick and Quick Operated Inversed Positioning LeverIntegral Ribbed Rubber Butt Pad with a Non Slip DesignIntegrated Quick Detach Sling Swivel Connectors on Both SidesWatertight Battery Storage for Two CR123A 3V Lithium Batteries / AA 1.5vFits Perfectly on Both Tubes - MIL-SPEC and Commercial Aftermarket!Made from Mill-Spec Reinforced Polymer Composite