I use pretty big oversize roundball, the slug I get is usually 1.7-2 X the bore diameter.

I haven't thought of casting lead for the slug, I do however know of a shop in SF that sells lead wire, that's likely how I'll end up setting my daughter's 25-25 stevens double rifle for bullets. Snip the wire, into the bullet swage, then into the brass.
I'll pick through the top drawer for lead pistol bullets and re-slug the Tolley to see if I come up with different dims than I'd gotten with roundball.

The loose rib comment, I'd likely by the time I'd gone to cast chambers whacked the barrels with a block of nylon to see what the sound was like. When I was a bicycle framebuilder that was how we'd 'first check' a frame or fork for cracks. A good frame sounds like a good frame. A crack will sound off in a very distinctive fashion, even if it hasn't showed up through the paint yet.

Oh, and I saw your report on your last safari with the Searcy. Sounds like you had a ball out there!

--Tinker