I wonder how corrosive unfired BP is in a cylinder over time?
It's not corosive at all. It's the fired residue that's hydroscopic but even that isn't nearly as corrosive as most folks make it out to be. It was the old mercury caps that were so corrosive that gave bp it's corrosive reputation. Now I'm not saying to let a fired gun sit uncleaned for 6 months or more but a few days to a week wont hurt anything. Pyrodex is supposed to be more corrosive than bp and I let a fired 58 Remington sit for nine days without any rust or corrosion. I found an original 58 Remington in a barn once with all six chambers loaded and capped. The gun was in pretty bad shape but cleaned up well enough to shoot. I pulled the balls and actually fired the powder out of a repro 58. The inside of the chambers were as bright and shiny as new ones.