This precision ground pin is used as a gauge to index the barrel nut on your AR-15 or AR-10 rifle. You simply tighten the barrel nut so it is close to the correct position, then slide this pin into the hole for the gas tube in your receiver. It will sit in the groove in the barrel nut allowing for a point of reference. If the pin is not centered on the barrel and pointing toward the hole in your front sight base, the barrel nut needs to be adjusted. The reason the barrel nut needs to be centered is when improperly torqued, the gas tube will be pushed to one side more than the other. This will cause the tube to wear prematurely, and impinge on the bolt carrier gas key.