In Feb., I posted a thread about a Youtube on firefighters burning ammo in different places to see how it reacts to fire. That is a training film now used by FDs all over the country. Wish I could have had just part of the ammo destroyed. It was sad.
Some departments back off. Some flood the area and don't worry about the ammo. A fire grows 7 times it size every minute it burns. So faster the atttack, the better the results. If it ain't poppin, you have time. If it starts poppin, duck.
As to a gun safe: At first, a safe gives some protection as the surrounding air heats up. The interior temps take a long time to match the fire outside the safe. And I mean a long time. The safe becomes an oven, cooking everything inside. Now a bullet in a chamber would have to heat up from the increase in the temp to the outside of the chamber. So more time to cook off.
So, for a bullet to cook off in a gun in a safe. Would not everything in the safe be already ruined from the heat? Grips, stocks, slings, gas in scopes, and anything that is combustible is turning to ash. Bullets in a mag would cook off before a chambered round. With that kind of damage, what would it matter if loaded or not.
After looking a several houses that had burned, we came up with a things to consider when placing a gun safe. If anyone is interested, I can remember most of them and will submit them to this thread if requested.