AR-15 barrel nut shim kit allows you to index the barrel nut so you have perfect barrel nut / gas tube as well as top rail alignment, within the recommended barrel nut torque ranges. Shims can be used individually or combined to get the desired thickness. Kit comes with (2) .001", (2) .003", and (1) .007" thick shims. The shim fits between the barrel extension and the barrel nut. Will not affect headspace.