I "had" some ammo in my storage building when it burned. I saw lots of bullet holes in the tin roof, melted lead coming out of copper jackets, melted buckshot. However I also saw ammuntion that survived and will likely fire. I also saw ammo that cooked off inside a big "fire resistant" gun safe, yet some ammo inside didn't cook. BTW that ammo going off inside the safe just compounds your problems during a fire, the only ammo in my safe now may be something in a mag........NO LARGE QUANTITIES.
I don't have a good answer regarding temperatures for it to cook off. I had another uninsulated safe (smaller) on an exterior wall that hardly cooked any off, the difference was the big safe was in the middle of the building, the smaller was at the edge. Plus the ammo in the smaller safe was closer to the floor. I'm thinking a bit "cooler" there.
IIRC Mythbusters showed solo ammo cooking and essentially blowing out the side of the cartridge case. Same ammo, same condition that is chambered will likely fire a bullet at a fatal velocity. Tightly packed ammo, for instance 30.06 in clips in a spam can, or AK ammo on strippers packed in a spam can are going to offer the opportunity for the tightly packed ammo to "fire" their bullets, maybe even with a dangerous velocity.....it just depends.
One other thing, I think that safe manufacturers have it backwards. They tend to have the safes designed where longguns are on the bottom and handguns are stored up higher. IMHO it would be best to have or install a low bottom shelf (it wouldn't be handy to reach) let the longguns stand on the top of that low shelf and put handguns under it, so they are lower = cooler. Dollar for dollar and for their size I think they should be on the bottom. By all means keep them in some sort of padded case. My fire was so hot it melted the dial off the door and the drywall was crumbled from the heat sort of looking like a jigsaw puzzle pieces. Guns stored in a nice padded soft case seemed to be insulated from the heat, the padded cases where charred and/or ruined, but the guns looked pretty good.
Lost a lot of ammo (in and out of the safe). Lost one handgun that was in a nylon bad on top of the safe. All the handguns can be cleaned up (soaking in kerosene as I type this). The longguns repair varies and IMHO dependent on the cost of the gun whether it's worth it or not. A Mosin-Nagant is < $100, it might make a decent project but it wouldn't really be cost-effective.
UGH on the whole thing......bad memories