I would say your initial wall thicknesses are too Low!!
G T Garwood quotes an average of .114" for end of chamber on English 12ga Guns from the mid 20th century. He goes on to point out some recent ( c@1970) European Guns that Average .084" but uses the yield point of 32 imp tons of superior barrel steel to demonstrate that this low thickness is at the bottom end of the safe range.( See shooting facts and fancies Ch34 for detail).
At 9" from Breech I like to see .040" and in the run ( 10-4") back from muzzles .025" will give sufficient thickness to reduce risk of denting. As noted above an increase in thickness is warranted in sub 12 gauge guns.
