The screw bbl pistols are quite common.
Remove the bbl with a 'key' or wrench that slips over the muzzle and down to the breech end. The small lug on the bottom of the bbl keys into the wrench and turns the bbl off. Most times you don't need the wrench at all.
The pistol(s) rarely have a front sight, so the issue of clocking it straight up is not there.
On a rifle or shotgun, perhaps so if it becomes worn.
Maybe a small front sight bead mounted on an adjustable 'ring' on a cannon type muzzle shape could take care of that and be disguised well enough. Just a thought
Aside from fumbling with getting the bbl screwed back into the frame at times w/o spilling some powder, the small pistol I had for a short time worked easily for me. More common to have the chamber in the frame I guess.
I'd drop the proper size ball into the 'chamber' (no patch) where it stops up against the start of the rifling.
Fill the remaining cavity behind it with B/P to level of the bbl. Screw the frame and bbl back together,,prime pan and you're ready.
The sealed breech with the ball up against the rifling is probably a plus as far as the power quotient goes, but at a few feet or yards, it probably didn't make much difference.
A rifle or shotgun made from the action would be different and neat to play with I think.