Black powder is different chemically from smokeless powder. When BP burns, it leaves salts as a residue. That's why prompt cleaning is needed to prevent rust. Salt is dissolved by water, so most BP shooters clean the bore with hot water and usually add some mild soap. My favorite is Murphy s Oil Soap mixed with the hottest water possible. Submerge the barrel, soak for a short time and then clean with brush and patches till clean. I submerge the nipple/flash hole and pump the soapy water up and down the bore using a wet patch on the end of the rod. Don't forget to clean the flash hole/nipple. The hot water makes the water evaporate quickly. Finish with a light coat of oil. I like Ballistol as it makes a great cleaner when mixed with water, can be used as a patch lube and used straight as a final oil wipe down. I like mineral based oils rather than traditional gun oils. Mixes better with BP. Don't forget to run a dry patch down the bore to remove excess oil prior to charging the gun. There are some BP specific solvents on the market, Birchwood Casey, Hoppes and others, that work well but soap and water is cheaper. Never had a rust or fouling problem with the above method.