Joe, I have seen .010 difference from top rib to bottom more than once.
At the Southern three years ago a member of our organization had me check his barrels wall thickness. Mine is a take-off of the Galazan gage and to do it right takes me 20 minutes or so going from top rib to bottom rib. Anyway I started 5 inches from breech and go every 3 inches till halfway and then go to 4 inches. Both of his barrels were almost concentric within .002.
I can go 15 inches and then turn it upside down with the breech up providing the choke area is at least.690 any tighter and it won't go over the rod.

David