I have a twist on storing ammo,
But depends on the ammo.
I use ammo cans, GI issue,
Use food grade silicone grease on the gasket, cheap from any restruent supply store, used to lubricate food processing equipment.
Petrolum grease will attack the rubber seal, food grade silicone won't.
This solves the moisture migration issues.
I buy rolls of 'No Corrosion' (brown, slick on one side) paper to line the cans for long term storage.
This stuff is fairly cheap, doesn't take much, and it won't matter if the ammo is stored for 20 years, it will come out looking like the day it went in.
Short term storage I simply line the can with what ever cardboard I have laying around.
The idea here is something between brass/copper ammo and steel box will stop the electrical process called the 'Galvonic Response'.
When dis-similar metals come into contact with each other (chip or rub in the paint) the two metals form a weak battery that causes corrosion.
You can also use the gas from a MIG welder, chases out the oxygen/moisture replacing it with neutral/non-reactive argon gas, both clean, neutral & dry.
The last resort is cooking a desiccant pack in the oven for two or three hours to dry it completely, then putting it in the can.
Cardboard, protective paper (removes oxygen and electrically insulates the can from the brass/copper) and argon from a MIG welder are all cheaper than the commercially available desiccant packs, not to mention faster...
If you are worried about the house/garage burning down,
'Fire Brick' is actually called 'Refractory Brick' is cheap.
Its the stuff they line fireplaces & chimneys with.
Line your storage place with refractories, and close up the space with refractories to give you several hours of fire protection.
This is the same stuff found in 'Fire Safes'.
When I remdeled the house, a large closet got refractories in the walls and a fire rated security door.
Before the remodel, it was just a steel copier cabinet lined with refractories & gun safes, now its an entire room, space between metal wall studs is filled with refractories, fire rated wall board inside the room,
Invisible, you can't tell it from the drywall in the rest of the house.
(Good idea for a safe room also, bricks will stop a sign post or 2x4 in a tornado.)