Here's the best way for long term storage of ammunition--or anything else of interest or importance....
PVC pipe--large diameter PVC pipe.
10" / 12" or larger inner diameter PVC. Schedule 40 pipe is great (white PVC). In the larger sizes, the wall thickness is slighly more that 1/4".
Large diameter PVC is easily available at plumbing supply warehouses--that kind that tradesmen and contractors use. Here cash is king--so walk in, know what you want, and wave cash... you will not be turned down.
PVC is easily cut with a hand saw. Just make sure your ends are square, this done by wraping a string around the pipe--measuring from the end for alignment, then marking with a pencil or marker. Cut along the line as you roll the pipe.
Your containers are then sealed using PVC (same schedule) end caps, plugs or screw caps. Make sure you use lots of PVC primer, before you apply liberal amounts of glue.
End caps are recommend because they provide a completely sure-fire seal. Allow a few inches of room between the end-caps and your stuff. This is because when you want to open it later--you tap the stuff to one end by tilting up the container. Then you saw around the end cap. Voila! It's opened.
I would suggest using lots of desecate (sp?) packs inside your containers. And before you seal it up--use a hair dryer on low heat... with it aimed into the container for a half an hour or so. This will drive out excess moisture.
Hidding your containers? Burrial of course. But not off your property--or property you cannot control. Crawl spaces under houses, sheds, etc.--sprinkled with lots of scap pipe of course to throw off metal detectors. Do not burry you container in ground that has a shallow water table--don't push it. Or on a potential flood plain--it may float up. Do not burry your container on ground that has vehicle traffice (tractors, cars, etc.) --you don't want to crush it.
If you need a BOC (bug-out-container) off property, then you have to be very clever where you place it. Again take in all the considerations above. Formost you need security. Un-trodden space. Unseen. Away from the obvioius places where treasure hunters with metal detectors would ply. No abandoned homes, ranches, etc. But it has to be accessable. This is the conondrum! Also... you need security when placing your container. You don't want to plant something under the watching eyes of a theif--who will wait for you to leave and dig up your goods. Put some thought into this process, with the thought of how you are going to re-locate your cache rain, night, day, snow or whatever. Also--if you should go down--make provisions for someone else to take up the flag.
Ammo cans--not good for burrial. Yes they are water-tight, but they rust. I believe a 50-cal ammo can will fit into a 12" dia PVC. ---this would be optimal. Ammo cans would be good in a walled-up hide-hole, false floors, etc. where fast availability is necessary. Also when storing metal items (ahem!) inside PVC--oil, oil, oil and seal in plastic. Break items down if necessary.
Storing ammo only--2" diameter PVC is readly available at Home Depot. A lot of ammo can be stored inside a 2' x 2" PVC container.
Sky's the limit using PVC--thank God for Dow Chemical.