If you have a traditional gun with a percusion nipple and side lock hammer, when you remove the cap. Take a roud ball or something lead and lay it square in the nipple and drop the hammer on it. It will deform the lead to match the nipple and seal it against moisture infiltration.

It's also a good idea to leave the gun outside in a constant temp. Going in and out warms and cools the gun causing it to suck in air and moisture with it which condences inside the gun.