I have seen a shotgun proved in Birmingham at 850 bar with a 2 3/4 chamber lenghtened from 2 1/2 and a barrel wall thickness of 83 thousandths at the muzzle end of the chamber.
My damascus barrel that I shot the 3 3/4 dram eq. with 1 1/4 oz load is 95 thou at the same spot. Not nitro proofed.
I still wonder how much do we increase the risc of blown up barrels with the higher pressure rounds?
If that Baker barrel was blown by only 6500 psi, there is nothing really safe to shoot.