Except for the hygroscopic powders it should not be a problem, I use Blackhorn and Pyrodex and leave them in for our muzzleloader season [3 weeks] and have never had a problem. I load with the gun clean and dry and unless it is an oil patched ball I cover the end of the barrel with a thumb cot and put an expired 209 in the inlines and a rubber nipple cover on the Traditional rifles. The best buck I ever got was taken in a downpour with a custom made Hawken 54 caliber muzzleloader.