A vacuum sealer like a food saver might work to seal out the elements. I personally have no reason to bury anything but if I was going to bury a rifle or similar object I'd oil the heck out of it seal it in a vacuum bag with some silica gel and then seal it up in a PVC pipe with caps and PVC cement. If your house is anything like mine there's no way to remove all evidence of firearms ownership. Got gun books, hunting books, trophies, antlers, game calls, etc. in the house? Do what you can to prevent them from getting the guns and put a rider on your insurance for them. I think the NRA offers supplemental insurance for firearms also. I would think burying the guns would be very inconvenient even if they were just below the surface. Dave P. is right leave a map for your family so they can find the guns in case anything ever happens to you.