My calculations show that 10 mm = approx. .39...correct? Which is what my Whitney would take. I do not know for sure what the size of the original moulds would have been.
I'd be careful if the chambers have been bored out to take more powder. That makes no sense. The gun is 150 years old to begin with, so making the cylinder walls thinner might be dangerous. Does the seller know for sure that it has been bored out?
Are you new to cap and ball shooting?