S&S Only,
The caliber of the Drilling not being marked is pretty common for guns of its age. About the only to accurately determine the nominal caliber is to "slug" the barrel and make a chamber cast. If you make a chamber cast to include enough of the barrel, you might not have to "slug" the barrel. Because of your lack of tools, you may need to have a local gunsmith or experienced handloader make the chamber cast for you.
Mike