Some years ago I had to pull the Cherry Corners Breech from a friend's Percussion Hawken; It had been shot a lot but always cleaned religiously afterwards with hot water and oiled with 'Young's 303 Gun Oil'. The way my friend cleaned; the tip of his rod and patch could not have reached into the Patent Breech - yet it was so clean; with no tarnishing or corrosion apparent. He always cleaned as I do today with the nipple still on and a piece of tube over it to direct the water away from the woodwork. The breech had to be replaced but that was not related to the way it had been cleaned.