The gun is and always was a cap lock, by the looks of it, and having a hooked breech.
So- powder first, then wad or wads, then shot, then a to wad, place a percussion cap the nipple and fire.
Alternatively, powder, then cloth/patched round ball, capped and fired.
I would suggest that guns needs to be cleaned up and checked very carefully, as well as likely needing a new nipple
before it could be fired.