A drop-in replacement buttstock for AR15/M16 carbines using mil-spec sized receiver extension tubes, the F.A.B. Defense GL-SHOCK M4/M16 Shock-Absorbing Buttstock is designed to deliver reduced recoil for improved comfort and control. Featuring durable, yet lightweight fiberglass-reinforced polymer composite, this stock is equipped with a ribbed rubber buttpad and a durable, silicon chrome alloy spring, drastically reducing felt recoil while providing positive shoulder contact to prevent slippage, even with body armor or modular gear. 2 QD sling mounts accepts any push-button sling swivel for maximum versatility, and a watertight battery storage compartment readily stores 2 AA batteries or CR123A 3V batteries for added convenience. Drop-in replacement buttstock for AR15/M16 carbines Fiberglass-reinforced polymer composite construction Works with mil-spec and commercial buffer tubes Designed for reduced felt recoil for fatigue mitigation and comfort 2 QD sling mounts Watertight battery compartment