Just added some more photos, one showing the current weight of the barrels at 1179g. Barrels are stamped at 1238.6, which means 60g of metal has been removed from the barrels since being proofed.

When I do a quick calculation using an average density for steel, the thickness of 60g of metal removed from the diameter of just one bore of these barrels would come to 0.072 thousandths. Meaning a negligible change in wall thickness from proof.