I would have him take a light cut on the face of the barrel nut and the receiver - just to true things up perfectly. You have it apart right now, so this is a good time.

Does your barrel nut have threads all the way out? If so, cut a 1/4" out of those threads on the tapered side (TOWARDS THE MUZZLE OF THE BARREL!!!!)....see the pics on the link BillPA listed.