This Griffin Armament M4SD Shim Kit includes stainless shims and a hardened steel alloy over-travel stop for your M4SD series silencers. 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 O'clock or bottom of the muzzle device.FeaturesStainless shimsHardened super alloy over travel stop