If I have a fire...they are all fried.
If you have enough invested that is over and above the cost of a steel safe, that’s the best bet.
Some of them have steel bars that go into the door frame, electronic code touch pad, can be bolted to floor or wall, are pry proof, too heavy to move and will resist fire up to a certain temperature for a specified duration. Which is better than having to show an anti-gun insurance company a pile of melted barrels.
Winchester makes some for a popular farm supply chain and often they go on sale at Christmas.
This is not the only place that sells them, but just a for instance.
http://www.tractorsupply.com/tsc/product/winchester-10-gun-granite-safe-with-electronic-lock
I only buy stuff on sale.
Ammo: harder to replace now if it were burnt or stolen.