I think it's an interesting topic. I always figured that if I were to do it I'd try to bury it 3' deep or more and I'd just put the gun inside a container filled with motor oil. Maybe I'm wrong but if it's sumerged in oil, the water shouldn't be able to get to it and when it comes out it'll be pretty well lubed up. I think it'd be neat to try this with one of those mossberg JIC shotguns that come in the storage tube. Not a bad package if you're into shotguns.
My thoughts are still to take an 8" diameter piece of PVC. Maybe make it six foot long. Glue a solid cap on one end and glue a screw out plug on the other end. Next get a section of 6" PVC. Solid cap on one end and removable cap on the other.
Dig big hole for first tube to go in vertically. Pack around it nice and tight up to maybe the five foot mark. Now take your smaller tube and fill it with whatever goodies you think you need.
Make sure that you wrap a length of cord around the smaller tube if it is going to be much shorter. Easier to get a hold of it that way.
Cap the big tube and cover it back up. Remember where it is at and all is good.
When the time comes that you need it, or just want to check it out, you will be happy that it is inside the other tube. More digging the first time, but much easier to remove the second as it slips out.
Lengths vary by need, but I would be sure to get below the frost line. Maybe pack sand or small stone around the top foot to allow easier digging and more water transfer.
As for the worry about leaking, you now have two caps. Besides I have used pipe dope on screw caps that are used in sewage areas with no problems for years. Unless the freeze/thaw creates a problem.