AAC's Barrel Shim Kit for timing the installation of your muzzle device. Don't risk misalignment by using a crush washer. This set is for any 7.62 mm applications. AAC's Barrel Shim Kit includes 6 different shims of varying thickness's for the purpose of properly timing a flash hider or muzzle brake on a particular barrel, offering a far greater degree of precision than that of any crush or peel washer while also being faster and less labor intensive to install correctly. Each shim is color coded according to its thickness, and multiple shims may be combined and used together if necessary in order to acquire the exact thickness required for proper muzzle device orientation.