Added some additional pics of the barrels:

http://s304.photobucket.com/user/djangofandango/library/?sort=3&page=1

About 1/2 an inch in to the chambers the walls measure 4.77mm/.188 inches - I agree that wall thickness needs to me measured through the forcing cones.

My thought was to get some 10 gauge gaugemates with the forcing cones built in provided there is still sufficient wall thickness near the end of the chambers and shoot 2.5 inch 12 gauge low pressure loads.