brybem. I tried exactly what you are thinking of doing about 40 years ago. The rifle went together pretty easily with the tang from another BP rifle of the same model as mine but in 45 cal.
The barrel was a 1-14 twist Remington 721 take off, with a 257Rob chamber. It was a heavy barrel so there was enough meat there to cut away most of the chamber and cut new threads for the tang/plug.
OK, here's where the problems started. First, I had to get a special round ball mold and a maxi ball style mold made up. Not a big deal because we had a marvelous machinist with lots of tool and die experience in the shop of the factory where I worked. He liked doing this sort of thing so a bottle of Pinch (his weakness) was considered payment enough. He turned out a couple of cherries and made up some molds out of old brass salvaged from molds for casting glass containers. Anyway, they worked well.
The next issue was powder. You will need to use very fine granules such as those made for handguns. In reality, even those aren't fine enough. You just can't get enough powder behind the bullet to get decent/consistent velocities. I had to cut the length of the barrel back to 18 inches to get it to work even reasonably well.
Now, I wanted to patch the round ball with cotton patches. This meant a ball that was around .200 diameter. The Maxi was of course slightly larger than bore diameter. The Maxi worked best but was a bear for lead fouling. Just couldn't get enough lube into the grooves to do a decent job. The ball was a problem because it weighed around 20 grains and at 25 yards wouldn't penetrated a 2x4.
There was a very good reason why the BP industry settled on .32 as their small diameter bullet of choice. .36 is even better and will likely do everything you want to do, including being almost ideal for squirrels and rabbits or grouse.
I won't try to dissuade you from experimenting. Have fun and learn.