Griffin Armament M4SD shim kit. This kit can be used with any Griffin M4SD muzzle device or an A2 nato birdcage compensator. Griffin Muzzle devices and silencers ship with this kit however if intending on mounting your M4SD series silencer to A2 nato compensators this kit is necessary to ensure perpendicular alignment and an overtravel stop for the suppressor. NOTE: The tab of the overtravel stop should be positioned at the 6 oclock or bottom of the muzzle device. Stainless shims. Hardened super alloy over travel stop. Package Contents: Griffin Armament M4SD Silencer Shim Set Overtravel Stop Griffin Armament GAM4SDSK: M4SD shim kit