I thought of that before too, and I think it work work swell. Not for me though, because...

A. Don't have the space to put one.
B. Have too damn much ammo/ammo cans for one. One of those boxes would be great for STORING large quantities for long periods, but I have way too many different calibers and loads. I'd be shuffling them around all the time to get at what I wanted.
C. My back said "Bending over and lifting up heavy-ass .50 cal. cans full of ammo out of a big steel box? Oh HELL no!"

So, I'm just planning to build some sturdy wooden shelves and enclose them with 3/4" plywood to build a big cabinet. I can put a LOT of ammo cans on a 12"x60" wooden shelf, and I'll have probably 5-6 shelves when done. I can quickly see what the cans are when on shelves (have labels on the cans), and can easily pull out the cans I want. When I get that side of the garage insulated and sheetrocked, that is.

Like this only lots bigger.