I use 8,000psi as my default British proof 20 bore loads in both 2 1/2” & 2 3/4”. Remington 20 hulls are 2 9/16” so I no longer trim them to 2 1/2” but I do for the rest. It all comes down to being certain that every shell I have is safe to shoot I every gun. I could go up to 9,500psi and still be safe but I do not for two reasons. Old wood cracking is more of a concern than barrel failure. The second concern is shooting 2 3/4” shells in 2 1/2 chambers would result in slightly increased chamber pressures so I err on the safe side.
