I'll second the "storing" a loaded rifle at the same temp as what you're hunting in. The quickest way to screw up a charge that's been at freezing all day is to bring it inside where the woodstove is cranked up. The metal will sweat and contaminate the powder with moisture. My garage is usually close to outside temps if I leave the heat off so that's where we leave our loaded rifles over night.
Sealing the flash hole or nipple prevents atmospheric moisture from tainting the charge there; and we use an "X" of electrical tape over the muzzle with one or two wraps around the tag ends to keep it in place, to seal the bore during crappy weather. A greased patch should seal the powder charge sufficiently but we use the tape mostly to keep rain, snow, dirt, crud, etc, out of the bore.
Our concerns are mainly "keep your powder dry", but damp blackpowder can and will rust steel.