Wet, or humid weather won’t create this problem. Moisture is the cure, not the cause. Very dry weather, and excessive heat will occasionally make dry crust at the bottom of the bore. But burned black powder is hygroscopic, and almost immediately after burning, begins to absorb moisture from the air, softening the crud. Patch lubes, exotic oils, household cleaners, and who knows what else, often contain components that have a low flash point, allowing them to polymerize at the breech where the charge is the hottest, and the pressure is the highest. The cure is a lube with a high flash point, that isn’t adulterated with a bunch of stuff that can’t take the heat and pressure. Bear grease, or oil, venison tallow, mutton tallow, all work well.
Hungry Horse