I bought a used "caplock/flintlock" convertible .50 rifle from a friend. He always shot it as a cap lock. I am a flint shooter. It is, however, the first flintlock I have had that had a patent breech. When I shot it with the cap lock on, drum/nipple, etc., I had occasional problems with the cap flash not igniting the powder. No side-slapping after charging got that fixed. Same when shooting as a flinter.
So I took it apart, removed the nipple, drum, and screw, and applied my bore scope. The barrel was clear, clean, and shiney; to a point. The patent breech was corroded and nasty. I spent the better part of an afternoon with drills, picks, pipe cleaners, and .22 brushes, with and without steel wool and JP bore paste, to make the fire channel, breech area, and breech smooth(er) and shiney(er). I contoured a breech plug scraper to the correct dimension for the patent breech. I have since used only real black powder, good caps and sharp flints, and have had no problems with ignition, wither cap or flint. But it must be kept clean, with an eye toward not driving "schmutz" into the works while shooting/cleaning. You gotta know your weapon and treat it right.
ADK Bigfoot