Cooey used to build a special model for Vets. I got a couple of them from an estate sale. Sold one to a local vet, complete with all of the accessories.
The receivers on these were purposely lightened, so they couldn't be used with high pressure ammunition.
The one I kept had broken parts in the receiver, so all that was usable was the butt stock and the barrel, which just happened to be "rifled" with a 1-24 twist rate.
The barrel easily fit onto a regular Cooey single shot receiver, which I just happened to have on hand and it would accept a 16ga shot shell.
I used a forcing cone to smoot over the sharp edges where the cylinder a Vet would use shouldered up against.
then loaded a 16ga shell with black powder and a patched ball. All went better than I expected. I did try some slugs, but they didn't work as well as the patched balls, as they were a bit loose in the bore for good accuracy.
Making up an insert for black powder, with a nipple for percussion caps wouldn't be difficult at all.