I loved my 12 and 10ga pipes. I would confine my roundball expectations to about half of that distance, completely work one barrel at a time, you might get a pleasant surprise.
I have used RS, P, and both grades of T7 for a long time in pistols, revolvers, and side hammer percussion rifles, 10, 12, and 20 ga shotguns for a long time now without any gripes re. performance or clean up.
It would have to be very Foster Slug like (think badminton birdie), they have been working in smoothbores (12, 16, 20 ga) for about 70 some years now. Solid conical in a smoothbore - save your powder...
Touch up may take the glare from the dings but they will still be there and visible, I would take the other screw out find a smooth flat surface and polish them (match the ‘sheen of the other side) out then cold blue and try to come close to surrounding area.
Soft pure lead does the best job on most deer size game. This was a .495 pure lead ball loafing along about 1600 fps, One shoulder was soop/cat food. Approx 60 yds.