Carl, you do not have to "take a thermos full of hot soapy water with you to the range". It may be a good idea, but it's often just not practical.

It is not the black powder combustion salts themselves that are directly corrosive, so much as the fact that they are extremely hygroscopic. And when the water gets into the salts, boy, then that is corrosive!

Someone is going to shoot me down for saying this (so what - as an old Germanicon saying goes: once you've lost your reputation, you can live as you please) but if you are away for some days and cannot clean it out properly, then a quick fix is to remove the nipple after shooting, while the gun is still hot (they can be vary hard to remove when the gun has cooled down) and then spray a good penetrating oil into the bore and the nipple socket.

That will save your rifle until you can get somewhere to clean it without becoming unpopular. But you must clean it properly before shooting again, as oil-soaked BP residues will clog up the touch-hole/nipple socket quite effectively!

Patrick