Funny thing regarding moisture, did you all know that IMR have a large container filled with IMR 4895 from the very first batch IN WATER that they periodically take out a pound, dry it out and test it alongside a new batch to see how the 2 compare?
I believe Olin have 748/760/296/780 all stored in a similar manner.
True story!
Most powders, not all though, are virtually waterproof due to all the deterrent coatings, additives and type, Double Base powders will NOT change their moisture content due to air moisture, but like all powders, they start to degrade if let to get very hot then cold then hot again.
Cheers.