Take a 5/16" x 2" long bolt (preferably one that the threads don't go all the way to the head) with 2 nuts and 1 lock washer. Take a 3/4" copper pipe cap and drill a hole in the center of the cap big enough to fit the bolt.
Put one nut on the bolt, followed by the lock washer. Now install the cap, followed by the remaining nut. Bolt threads should be flush with the top of last nut. Tighten well.
Now, cut the head off the bolt and file off any burrs. Pack the cup well with steel wool to the top of the "cup" of the cap.
Chuck the gizzy in an electric drill or drill press and hold the mouth of your cases flush against the steel wool in the cup. Voila! Takes the burrs right off the case necks in short order! Nice and square too.