Schuster MFG Barrel Nut Indexing Pin For AR15/M16 This pin is used as a point of reference when positioning the barrel nut. Simply slide this pin in gas tube of your receiver and make sure it sits in the groove of your barrel nut and properly aligned with the hole in the gas block. If the gas tube is improperly aligned, premature wearing will occur on the gas tube and the bolt carrier gas key. $13.00