Perhaps I am missing something, not uncommon, but you don't need a chamber casting, you need to slug the bore. Easily done by taking a cast lead ball or bullet (I last used a small fishing sinker that I tapped to about 3.3 dia.) and gently tap it into the muzzle with a brass hammer. Then poke it out with a rod from the breech and see what your diameter is with calipers.
FWIW my 1939 T&S Nimrod drilling slugged at .318 and shoots the S&B 196g JRS stuff into 3" with iron sights, or at least did 10 years ago when my eyes were better.
All that S&B stuff is steel jacketed BTW.
