This is a bit dated, but I plan to answer it anyway. Pack in the mid 1970's Pyrodex was the only game in town if you couldn't find Real Black Powder. I picked up my first can of Pyrodex in 1976. I would shoot it out of my T/C Hawken. I was trained right when it came to BP clean-up, do it asap with HOT Soapy Water. Dry it out and oil to keep it from rusting. The key is VERY HOT Soapy Water. The hotter the better. The metal gets hot and the water evaporates. A few dry patches down the bore and then a light coat of oil. Keeping this practice keeps my guns rust free. It has only been the last few years that I've heard of Pyrodex being corrosive as acid.
As I've said. Clean as soon as possible after shooting. Within hours, not days.