If you are buying ammo with the intent of storing it for a while, then there are a few major things to consider to ensure it's longevity:
- Avoid excessive, prolonged heat
- Avoid excessive, prolonged cold
- Keep it cool
- Avoid wild temperature swings
- Most importantly, Keep It Dry
For longer term storage, it is recommended that you store your ammunition in ammo cans with a rubber seal. The Military learned this a long time ago, which is why they started making ammo cans. For additional longevity, insert a moisture pack (desiccant) inside of the ammo can which will help keep humidity away from your prized ammo. Also, every time you open and close your ammo can, it is probably a good idea to switch out the silica gel packet as new moisture will enter the can.
5 gram packets are designed to cover 648 cubic inches of contained air space (enough for one 50 Cal M2A1 ammo can).