Not sure but I seem to recall proof loads are 2x safe load. Its St Pats day and getting late and my brain cell just had a bath so I'm probably wrong.
The beauty of black powder is its pretty hard to overcook it, go over the top and the excess just gets blown out the barrel unburnt.
Looks like a nice bit of kit.
“If a cricketer, for instance, suddenly decided to go into a school and batter a lot of people to death with a cricket bat, which he could do very easily, I mean, are you going to ban cricket bats?” :- Prince Philip said after Dunblane