The ignition temperature of typical gunpowder is around 800F. It varies a bit with exact formulation (some exotics can be as low as 300F).
The ignition temperature of paper is famously 454F. Well... not exactly. It makes for a dramatic book title anyway. The ignition temperature of paper is typically around 450F; it varies with the exact type of paper, but 450F is a good rule-of-thumb number for typical paper. UL assumes that all fire-resistant containers are for the keeping of paper business records. As a result, the standard test is how long can the container keep its internal temperature below 350F, 100F below the ignition of paper. 350F is how hot UL thinks the inside should get. 350F is a lot lower than 800F. So, I withdraw my previous comment and now think that it is ok to store ammo in a fire-resistant container.
BTW, UL set the 350F number based on the preservation of paper business records. I suspect that some of us keep other things in our fire-resistant containers... steel things. The melting point of steel, of course, depends on the exact alloy, but it's over 2500F. But, knives and guns can be ruined by much lower temperatures because these things are made out of heat-treated, hardened steel. The annealing temperature, the temperature above which that hardening is undone, varies depending on the alloy and the heat treatment process used, but around 450F is not unusual. By curious coincidence, this is about the ignition temperature of paper. So, UL's plan to keep safely below the ignition point of paper works well for the preservation of hardened steel objects too.
If you have a fire-resistant container in your house, there are some simple precautions you can take to improve its performance in a fire.
- Position it on the ground floor preferably on a hard-slab. If the container is on an upper floor, the floor under it could collapse and the container -- which is often heavy -- will then fall quite some distance and the fall could break open the container.
- Position the container on an outside wall. In a worst-case fire, the hottest spot is typically in the middle. We want the fire-resistant container as far away from that hot spot as possible. Also, positioning along an outside wall will allow fire fighters to direct cooling water onto the container.
- In the event of a fire, inform the fire fighters of where your container is and they can direct cooling water to that area to keep the container as cool as possible.
Most fire-resistant containers have a heat-activated seal on the door. The heat of the fire activates this material and the container essentially calks its door shut, sealed tight against outside water. So, the water from a fire-fighting effort won't get inside.
Many fire-resistant containers also have a thermal re-locker mechanism. This is a permanent lock which can not be unlocked from outside of the container.
It's activated by the heat of the fire and permanently locks the container shut. Once the container is recovered from the rubble of the fire, it will have to be destructively cut open by a trained professional. But, if the opening is done correctly, you should find the contents in good order.