IMO, the BEST way to store ammo is in USGI ammo cans. This is one of four shelving units in my gun room; and there are other GI cans on wooden pallets on the floor:
USGI steel ammunition box specifications reflect such considerations as container volume, ergonomics of loaded container weight, strength of can for stacking when loaded or empty, drop test, seal reliability, and safety in a fire. If it works for the US military, it works for me. All of my ammunition is stored in USGI ammunition cans.
Be sure to select those that are dry and completely rust-free inside. Slight surface rust outside can be addressed with a wire brush and more paint, but inside all you want to see is dry dust and OD green electrocoat.
Ammo can seals are viable if the can "burps" when opened. Slight changes in atmospheric pressure will result in differences in air pressure inside any given can vs that outside the can. It may be higher or lower depending on the atmospheric pressure where and when the can was last opened and then closed. That said, a seal may still be viable if the can doesn't burp, because a shopper ahead of you may have opened that can the same day. The place I get my cans is about 900 ft lower in elevation than where I live. Without fail, the pressure inside the cans at the store is higher than that at home, and the cans always "burp."
Noah