There's one way I would bury a firearm that I wasn't going to need instant access to. I've thought a little bit about it, and this is what I've come up with, IDK if it would actually work. I would seperate the stock from the receiver etc, I would put the barreled action, bolt, any other metal parts into a 6" PVC tube with a glued cap on one end. I would then fill the tube with synthetic motor oil, filled as full as I could. Then glue a cap on the other end. Then I would insert that sealed pipe into either an 8" or 10" PVC pipe with a glued cap on one end. I would then fill that tube with silica gel, put a PVC saw in it, and cap the other end with a gasketed screw on cap.
It's probably total overkill, but I think it would work.
What do you guys think?