F.A.B. Defense M4/M16 Shock Absorbing Buttstock, Fits AR Rifles, Black FX-GLSHOCK The revolutionary patented GL-Shock recoil 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 AR-15. Additional features include the ability to be used with either a Mil-Spec or Commercial diameter buffer tube" a non-slip rubber butt pad" water resistant battery storage compartment and (2) QD sling swivel socket points.