As for the breech plug fit it depends on who made the barrel, Rice are spot on, Colerain almost never properly fit, Burton, perfect and of course GM, they don't do plugs so you have to do it yourself. That is all I have experience with.
Take out the plug, put a little Prussian blue on the face of the plug and screw it back in to the witness mark, pull it again and see how well it marks the breech face. You need a good seal all the way around.