It is a BP gun.

Mickey, as you know alot of BP guns have rubbish bores and are worn
all over. I blew up the photos and did a couple of other things that allow me
to see the condition even though the photos are not great.

It looks like one of those sleepers you see that have been covered in oil
and left in storage.

A good black powder gun in good nick are getting harder to find.

500 Nitro