While we're on the topic of 8g guns, this is a Twist barrel (what could they have been thinking?!? ) British single originally 8g that was reproved 1925-1954, surely after honing as the bore is now 7 gauge - .873-.887. Chamber is 3 1/4".

Turn-of-the century British 8g loads were 6 Drams “Schultze” Bulk Smokeless and 2 oz. shot, at 3.66 tons = about 11,200 psi. Proof was with 18 Drams B.P and 2 1/4 oz. but I don't have the pressure - typically 30% higher than the service load pressure.

It passed Nitro proof for 2 1/8 oz. shot.