Be sure and leave the ammo in the boxes they come in and then put them in the ammo cans or if you want to dump the ammo in line the cans bottom and sides with card board. I have been told that the ammo against the bare metal can cause a chemical reaction that can cause the ammo cases to go bad. I do not know that for sure but I have been told that.
The ammo cans I have been seeing lately are heavy-duty plastic with a neoprene seal. they have the advantage that they don't rust or corrode. If you go to Cabela's Outfitters (a national chain), you can buy quantities of ammo in what look like .50 cal ammo cans at a decent price. They have most rifle and pistol ammo packed this way.
Military Ammo cans are painted inside and out with a thick coat of paint. There is no bare metal unless it's been removed. Even though it's painted I still leave them in their original box or in case of loose reloads I use plastic boxes. I do have two cans full of factory loose 9mm in ammo cans and I'm not too worried about it. The cans are air tight and have a good coat of pain on the inside. Military Ammo cans are designed for one thing and one thing only, store and protect ammunition.