I use old (non-fireproof) file cabinets filled with ammo cans.
If you find good ones, they're not at all flimsy, you can fill them with ammo, and you can get them for practically nothing.
They never have the keys, but you can buy replacement locks (keyed alike) for very short money.
I see file cabinets all the time in the "free" section of craigslist.