Fire is only one threat to ammo, water is another, and theft is a third.. My advice would be to bury caches made from PVC pipe for long term storage: Cheap, easy, and secure. Well it's secure as long as you can keep your mouth shut as to where you buried it. Some folks bury their caches under a patio made from pavers, decommissioned machinery or cars, shed floor, etc. -- that's helps to find it again when SHTF.
Larry