In the original boxes, then I vacuum seal that. My roommate loads his mags, then vacuum seals them. Either way, it works, and they hold their seal.
If I buy bulk ammo that is in one of those tins, or sealed, I just don't open it, and keep it stashed.
Ammo stays in a place where the temp stays constant.
America has fought wars across the globe, because we kept our firearms while others let dictators take theirs...