Not to put too fine a point on it,
But you posted your question in WATER: Collection & Storage.
I don't think Ammo cans are the best way to store water!
If you need to use big locks on something as small as an ammo can, then you are storing it in the wrong place...
A Good lock or two on a storage unit or a shipping container (ShipCon) would be fine,
But if you are trying to secure something small enough to fit in a ammo can, you should just hide it for security.
'Storage' can be defined into two basic categories, Short Term Storage & Long Term Storage.
Short Term Storage should be things like the stuff you use at home that needs to be rotated, like food, medicine, first aid and paper products, ect.
These relatively unimportant things should be stored in the home and rotated regularly.
Short term storage at home for some mildly important things should be done in fire resistant, and secure locations, like a safe that is bolted to a concrete floor or wall.
If your needs run to storing small, but expensive items for short term, then HIDE THEM!
Out of sight, out of mind.
I would never consider leaving cash or other valuables in a safe or other easily found lock box in the home...
Find a 'Cubby Hole' or make a 'Stash Hole' somewhere and hide your small be valuable stuff there.
Long Term Storage should be things that won't 'Parish' or things you don't need the entire world knowing you have.
My Long Term Storage consists of important papers, insurance, birth certificate, orignal Social Security card and reports, investment reports and certificates,
Family heirlooms, like my collection of ancestors pocket watches, expensive jewelry I've accumulated through one trade or another down through the years, stuff like that.
Some people want to store beef stew, bullets and bandages long term...
If you have anything you want to keep out of sight, that can't be tracked long term, then consider digging.
Burying valuables has been a long term and time honored way to keep them safe.
Something as small as an ammo can will bury quite well if you prepare the contents and the can correctly...