Most period guns need low pressure loads more to protect the wood than the barrels. What is a reasonable load was more the question than is the gun safe to shoot. I am sure that gun has had more than a few loads over 7,500psi shot in it over the years. For many in the US it has only been in the last 20 years that low pressure became easy to find or make for many shooters. How many shooters use to think that the height of the brass was an indication of pressure? Too many I am afraid.
I would measure the chambers. Perhaps honing them had elongated the forcing cone area or some idiot had a 2 1/2" chamber reamed out to 2 3/4" or more. I would want to know. If you are going to spend thousands of dollars on guns you might as well spend a few hundred dollars more to have measuring tools on hand and learn how to use them. Many on these guns have been altered over the years and you need to know it sometimes.
