Question
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Desiccant canisters help but in a leaky garage safe at 60% they'll saturate fast. Rod + a couple desiccant packs is the combo I run.
Ammo cans with a fresh desiccant pack and a good gasket are basically a time capsule. I've got 15 year old stuff that looks brand new.
Modern brass-cased ammo in sealed factory boxes is more resilient than people think, but primer sealant isn't forever. Getting that number down to 40-50% will let you store it for decades.
A Golden Rod is the single best thing you can add to a safe. It keeps the interior a few degrees warmer than ambient so condensation never forms. Cheap insurance.
Also get the ammo off the bare metal floor of the safe. Even a piece of wood or a rubber mat helps with the temperature swing condensation.