If you are in the field or for any reason unable to do a complete cleaning until later, at least do what they did in the civil war with their revolvers. They would heat up a pot of water with lye soap in it and just dip their entire gun in it, shake it around a bit and then let it dry out. That will at least dissolve and get a lot of the fouling out. In other words they gave it what is known as a "whore bath", until they could clean it more thoroughly later. I don't recommend that though as we are not usually in the kind of situation where we don't have access to our cleaning tools and can't do a more thorough cleaning immediately.
I always RELIGIOUSLY clean my BP guns right after shooting. It was enough of a pain to chase rust on my old blue steel BP revolvers even when I DID clean them immediately, (that's why I only own stainless or nickel BP revolvers these days, and I still clean those immediately too). So by neglecting cleaning immediately I certainly wouldn't want to help the rust get any better toe hold than it already will. The only blue steel BP weapon I have is my CVA mountain rifle. If possible I always take soap and a bucket and water with me to the range or wherever I am shooting. If I'm in the field, I'll use canteen water, ditch water, any kind of water even if I don't have soap. I always also carry a small can of oil and cleaning patches and rags and a knife to cut cleaning patches from the rags if necessary. For my CVA mountain rifle I already have a ramrod attached and pump the water through the barrel just like was already posted by another poster. For my BP revolvers I carry a compact cleaning kit with several rod brushes. I totally agree with the previous posters that BP weapons should be cleaned as immediately as possible after firing to preclude any possibility of corrosion.
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