It's hard to build an ar correctly without one of these - Midwest Industries Upper Receiver Rod

If you don't have one, before changing out that barrel or installing a new barrel, get one. It takes all the pressure off of the notch and barrel pin, it lines up the barrel to accept the bolt lugs and it does not stress the receiver as other rods without the top sail will do. For building AR's or changing out barrels this is a must have item.