Once with an old mauser, I discovered that to remove old dirt it was best to use different methods.
First I gave him two liters of boiling water, then a kind of cleaner, which I had lying around. Finally hoppes 9. Between each cleaner, a stainless steel brush, with very hard hairs, that I bought a long time ago on ebay, that was the best method.
Some friends leave the barrel one day with mercury inside.