OK guys, think this through. Black powder, unburned, containes saltpeter. This is chemically a salt, and salts are hydroscopic. That is, they absorb water. Water containes oxidizers (oxygen) and this causes rust.
Therefore the rusting properties of unfired black pwder will be dependent on the amount of water available for the salts to absorb and hold agianst the metal. Here in the east, that's typically a lot. In Arizona or the Anza Borrego Desert, not much. I would worry much less about a weapon kept loaded in a heated and air conditioned house than one kept in a garage, but I know how damp our house gets at times, and that's all it takes, one time, to get rust started.
I don't keep my BP guns loaded, and I keep my BPCR cartridges sealed.
Bottom line, Black Powder doesn't cause rust, oxygen does and it's found in water. Even that's not completely true, I'm no chemist, but it makes the point.