So the powder itself is a moisture draw!
I wonder if this is true re: all powders, including the substuitutes in pellet form.
And for the record, I don' t usually leave my BP weapons loaded for longer than the season....but it was a very odd year for me. I took the precaution of removing the primer of course, but also put the gun in a hard case and locked it case and all in a cabinet in my out building, which was also locked.
I' ll bet standard Black powder with patch and ball would be more of a moisture collector.