Sealing the muzzle with a bit of tape is good to keep the rain and small twigs out of the bore. It is especially effective for hunting the thick cedar swamps where loose twigs and other flotsam readily falls off the trees and somehow finds its way into actions, bores, and down the back of one's shirt.....

Another very effective method I've seen employed is to use a non-lubricated condom...... The lubed ones can be hygroscopic and could promote rust......... The other advantage of using condoms is that when you run into one of those remote northern girls, you can get her all giddy and worked up by complementing her on her lovely smile and "nice tooth" and voila, relocate the condom from your rifle's barrel to your own, and you're set!!! Like the boy scout's motto suggests: "Be Prepared"