A fellow who restored a large number of original rifles told me he would take a rusty percussion barrel and submerge the breech in a container of water and leave it there for a week, at which time the nipple could be removed. He did the same thing with antique locks, where he could remove original rusted screws, clean and re-use them. The water would turn the rust in the threads to a black dust. His explanation was water put the rust there and water will remove it. I tried it myself only once on an original 1858 Remington revolver and all the nipples came out, at which point I cleaned, oiled and re-used them.