I did a quick test with 3 different 209 Primers T7's, Win W209's and some Remington Kleen Bore 209's. I put them in a small bowl of water for 10 minutes or so and tried them, they all fired. So if your breech is sealed pretty good and no moisture makes it past the Primer, and you have your Muzzle covered, you should have no problem with moisture.
Like I mentioned befor earlier above, I have Hunted with the same load in My MLer for 3-4 days in the rain with my Muzzle covered with Masking Tape and she fired just fine. I have never had one not go off using the tape on the end of the Muzzle.
(BP)