As far as keeping ammo, papers, chemicals in the safe, you need to be cautious with the temperature range within. Should you experience a sustained fire, a safe is basically an oven and many products are heat sensitive. I cant remember what the ignition temperature was, even in absence of flame, but it seemed pretty low compared to the external fire.
I've read/heard of horror stories of the safe appearing to have done its job from the exterior, post-fire, but upon opening the safe, most everything was damaged/ruined because of the other materials that the homeowner had inside that were subject to sustained heat.