I guess I don't need me to remind you that black powder is corrosive. If you don't wash all the corrosive salts off of the metal it will rust. But all is not lost it normally doesn't rust much over short term, just brush it out using flowing water to flush out the salts. Dry the barrel out with some dry patches & lube it to protect & seal the metal surface. Then if you don't see any pitting on the inside of the barrel, take it to the range to check if it is still zeroed. If your gun is still on zero it should be OK.