Since you already have Cerrosafe and evidently know how to use it, you can get a very accurate measurement of the bore dimensions by reversing the chamber casting process and casting the barrel at the muzzle. Simply push a patched cleaning rod from the breech to within an inch or so of the muzzle and pour in the molten Cerrosafe. Once the casting has hardened, it is a simple matter to push it out using the cleaning rod. No hammering involved.