“Sir Jeff” at Survival HQ writes …
Close to 4 years ago I placed this cache. The gun was lightly oiled and placed with a desiccant pack in a mylar pouch. The magazines were also given the same treatment in their own mylar. Ammo was packed in ziplocks with desiccant also. Then a few extra desiccants thrown on top, the seal of the ammo can greased, and it was shut. The can was wrapped in a heavy garbage bag.
There were no failures whatsoever. No rust spots or discoloration. Even all the brass looked new.
Mylar seems a good idea.
[ Many thanks to Todd for emailing me the link. ]