In my opinion, British Proof is a load of manure once the gun exits from the proof house. Brits are famous for grinding the wall thickness from the outside, leaving the inside "in proof". A hundred dollar wall thickness gauge is worth ten times what a proof stamp is worth. Also in my opinion, two identical shotguns, one original 2 1/2" chambers, the other drilled out to 2 3/4", if fired with progressively stronger 2 3/4" bullets, the drilled out gun will burst first. Of course, the drilled gun will recoil less.