If you've got alot of factory ammo in boxes a stackon brand sheetmetal locker with shelves would do well.
If you've got a really big stash of ammo then segregate it by caliber into 50 cal ammo cans. Rig up a padlock for the latch on the can and maybe even string a heavy chain through all the handles and anchored to the floor. That would slow someone down a little bit I think.
I store all my ammo in various 30 cal, 50 cal, fat 50, 40mm, nvg, etc cans. I use these little reusable hydrosorbent packs in each can. They are blue when dry and turn pink when they are saturated. Put them in the oven at 250 for a couple hours and they are ready to go again.
http://www.midwayusa.com/product/799452 ... t-canisterAnd I shoot Swiss GP11 ammo that was made in 1979 and it shoots just fine. Keep ammo cool and dry and it will last darn near forever.