I don't see why ammo would need to be locked up. It is not going to do anything by itself. We never had an issue with our kids, or when I was a kid, by having ammo stacked on shelves or in unlocked cabinets. I have way more ammo than would fit in even a large safe or two. I have one HD shelving unit loaded with ammo cans of ammo and components and more ammo boxes on another shelving unit and a couple of cabinets. I keep some HD ammo with the guns but all the range and reserve ammo, and components are on the open shelves.
I agree, I just store my guns, extra ammo, laptop computers, and other stuff locked up in case of a thief entering when I'm not home.
I would like to have a fire resistant safe, but I would need a MUCH bigger house to put the darn thing in. I have a heavy gage steel gun cabinet with drill proof locks, but it is not fire resistant.