Barrel Shim Set to be used to correctly orient a timed muzzle device. This set is used with YHM 5.56mm Q.D. Muzzle Brakes. This shim kit has an inner diameter of 1/2". SPECIFICATIONS Included with this shim kit are shims with the following thicknesses: 2x 0.002" 3x 0.005" 1x 0.020" Mfg: Yankee Hill Machine Co., Inc.