THH,
I have left mine for the whole season w/o a shot. BP or pyrodex is not corrusive until fired. I always use a ballon over the top of the bore or a latex glove finger I cut off. This keeps any mositure out. I also take bore butter and swab the bore above the load if it does get wet out.
When cold I put my rifle in a padded gun case (cloth type) and bring into the porch until the load has warmed up. NEVER HAVE HAD A MISFIRE! At the end of the season I'll do a burn out and then clean'um up.
SHills