Shortgrub,
The easiest way would be to make a chamber cast and compare the dimensions to the cartridge you think it is. Just remember a chamber will be a little larger than the intended cartridge( from a few thousandths of an inch to more than 15 thousandths. Old cartridges vary the most). Cerrosafe is available from suppliers of gunsmithing tools and is the most recommended material for this. It is a little expensive, but is reusable over and over again. You should have a gunsmith check that firing pin( checking the whole rifle wouldn't hurt).
Mike
