A lot depends on the situation surrounding the storage.
If, for example, we're looking at a post-apocolyptic scenario (my favorite), were weapons have to be buried to be safe from thievery, you need more prep and securement.
If, on the other hand, you're able to leave your weapons in a "bunker" of some kind, something that keeps out the rain and is relatively secure, you can get away with less.
Are you looking for something that would allow the weapon to operate as soon as you can lay hands on it? Or, are you willing/able to spend an hour or more cleaning the goo out of the system before you can safely fire it?
The catchall that I prefer is to pack the internals with cosmoline or a heavy grease that is hydrophobic (repels water), then slide it into a plastic bag and vaccuum-pack it. Once that is done, it goes into an appropiately sized PVC (poly vinyl chloride) pipe that is sealed on both ends with a glued on cap. Said time capsule is then dropped into a pre-bored hole, resting in a vertical position to minimize the metallic signature, thus making locating with a metal detector more difficult. The general area surrounding the cache should be strewn with iron filings or finishing nails to, again, fuddle attempts to locate by nefarious characters.
If you need more ideas on survival, weaponry, caching, or whatever, do let me know. I'm always working on something.