Diameter of the pin is of large importance. If a black powder fat pin, the converting to a hornet is something I wouldn't advise.

The original calibre could have been anything from .22 cal RF., on up to the .30's. As it appears to not have been modified, perhaps it always was a .22RF? Boring to 5/16" is normal for a liner, which would give the appearance of being larger bored than it was.

Just a thought.