How should I store ammo for the long term?

Stored right, modern ammo lasts decades. The enemies are moisture and heat. Use sealed military-surplus ammo cans with good gaskets, toss in a few silica-gel desiccant packs, and keep them somewhere cool, dry, and dark — not a hot attic or a damp garage. Label cans with caliber and date and rotate oldest-first. Keep it locked away from kids and separate from firearms where your local law requires.