Seems to me that a seated bullet, regardless of type, should isolate the charge from moist air pretty well. When hunting in rain or snow, I like to tape the end of the muzzle to keep snow and liquid water out. Traditions also sells what are basically condoms, for the same purpose. Only other avenue for moisture to get to the powder is through the flash channel. When hunting, the flash hole is pretty well sealed by a cap or a 209 primer. For caps, Traditions sells rubber tubing of just the right size that can be used to seal the cap on the nipple. Those make the moisture barrier even more secure. With my sidelock uncapped, I like to put a piece of soft rubber over the nipple and lower the hammer onto the nipple to hold it in place and thereby provide a good seal. A piece of a wide rubber band works well. Using the Traditions stuff mentioned above and a rubber band when my Renegade is uncapped, I've kept the same powder and bullet in my Renegade for well over a month of hunting in cold, snowy conditions, with the rifle brought inside at night, with no problems.
