Griffin Armament Shim kit 18x1 or 3/4x24. Shims are necessary to provide precise perpendicular alignment of the muzzle device with the thread shoulder of the barrel. We do not recommend the use of anything other than shims for installing muzzle devices for silencer mounting applications. NOTE: All Griffin Armament Muzzle Devices include the necessary shim set for installation. Package Contents: Griffin Armament Griffin Shim Set Griffin Armament GABBSK: Shim kit 18x1 or 3/4x24