A gun safe will not protect ammo from a fire, the temp will get high enough to set off the ammo.
Safes keep the temp down to a certain point, fire proof is a misnomer, do your research.
I know a man that had papers, money and ammo in one, had a fire. This man was a LEO,
is a firearm instructor. He lost everything in a fire, had a good safe and thought he was safe from loss
due to fire.
Just understand in a fire you will loose ammo, paper, wood and plastic.
Some safes use dry wall like panels inside for a fire barrier, these panels
have a bit of water in them that turns to steam to slow down the heat from getting to high.
I feel there is no way to "fire proof" your stuff, just slow the heat down till you get the fire out.
The best thing to do is dont let a fire start (I know that is a stretch).
If you have the money, build a concrete vault, 12" block, filled with concrete,
poured concrete roof and a vault door.
Buy cheap and you will pay in the long run.
Put only firearms in a good safe, no paper or ammo.