Fire-rated ammo storage cabinet. Two reasons:
1. Reduced likelihood of ignition in a house fire, and massively reduced probability of further fueling a fire if ignited.
2. Our house is becoming a gathering place for our kids friends, and while I fully trust my own kids to never mess around with ammo, I'm not always as sure about their friends and classmates.
I ended up using a fire rated chemical cabinet, after doing some research and watching videos of their testing with highly flammable chemicals, which is essentially what ammo is. I used to shoot trap, and probably will again when life calms down a bit, so I bought something large enough to store a dozen flats (250 rounds per flat ~3000 x 2-3/4" shells), plus all my various high power rifle, pistol, etc. The likelihood of ammo reaching ignition temperature in this safe, which is sitting on my concrete basement floor, seems pretty low. If it does manage to reach ignition temperature, it is designed to contain the fire, safely venting thru baffled vents, like a muffler on your car.
Now I sleep better at night, when the kids are having a slumber party in the basement rec. room, just adjacent to the room where I store most of our guns and ammo. If anything ever happens in this house, I can assure you the word "arsenal"
will be used somewhere in the news headline, but at least they won't ever be able to accuse me of not keeping my **** locked up.
There's also a fingerprint-activated pistol safe in the bedroom, for the one or two items kept at-hand for home defense. It opens in less than 2 seconds with my wife's or my finger on the sensor.