I maybe can shed a bit of light on this subject. I have hunted with both traditional and modern firearms, but a couple things remain constant no matter what you choose. Black powder fouling and its substitutes to one degree or another, all like to absorb moisture, IIRC the term is Hygroscopic, or some such. An example, Goex fouling left to the atmosphere in the pan of a flintlock will turn glossy with moisture in just a few mintues depending on the humidity level. The powder itself is not so bad, I've hunted up to 4 days with the same load in my flintlock using a toothpick in the touchhole when I wasn't hunting and a borebutter patched round ball in the bore and not had even the slightest hint of a hangfire when I finally did touch it off. If the powder is kept away from moisture, it'a a pretty safe bet it will work fine, and I've never had rust in my bores. Same applied to my inlines and caplock rifles, sealing them is a lot easier, when using conicals or sabots, I use a piece of duct tape on the muzzle and a piece of surgical tubing over the cap to keep moisture out. If you fire a fouler, you won't have a problem IF you keep moisture away from the bore. I could go on with more about the merits of keeping your firearm shootin in the wet weather but that's another subject. The basics are the same. In a nutshell, the fouling isn't corrosive unless it has a moisture source. This is one reason why I like borebutter, it won't ruin the powder charge like oil and it protects the bore when the powder and bore meet.