Ah Ha.......it's a single shot.....I was under the mistaken impression that it was a double rifle! So no worry about regulation with an odd cartridge conversion. The "100" on the proof mark means "100 bore" or essentially .360 under the 1875 proof system.

Well; I'd get it going one of two ways:
1) Clean up the leads to the rifling and form some brass to fit your chamber, and us a cast lead "express bullet" to suit your twist. Easiest and least expensive.
-or-
Reline and rechamber to a small easy caliber like 32-40 WCF or 9.3x72R

Definitely NOT a hotter modern smokeless cartridge.
Have Fun.