* Friction lock performance may vary depending on the host receiver extension's dimensional tolerances.** Specifications based on standard-length (flat-back) M4 Carbine receiver extension and may vary depending on tube manufacturer.Standard Rubber Butt-Pad, 0.30" weight: 0.08 lb. Optional Extended Rubber Butt-Pad, 0.55" (weight: 0.11 lb.) or Enhanced Butt-Pad, 0.70" (weight: 0.13 lb.) may be used with either Mil-Spec or Commercial-Spec STR stocks.