You know what's in soap?
Yep. Miraculous stuff that cleans BP and lube residue. Been using a little dish soap in hot water for all my years shooting BP and it helps to cut through the fouling. Just hot water never did the trick for me, and dish soap leaves no residue when you rinse with more hot water. I've noticed no bad effects, and it has saved me time. Try this trick. Pour a small pile of black powder on a flat rock. Put a little gob of beeswax on the top of it. Light it off with a match. When cool, rub the palm of your hand on the black goo. Then go try to wash it off with hot water. Post results...