Fire proofing involves three separate things: exterior construction, fire load, and building design. Solving one aspect of this triangle doesn't work.
For example, a car's exterior is usually made out of metal. Steel doesn't burn. However, the paint on the steel does, as well as all of the plastic and rubber in a car. Bottom line is a car burns really well.
So let's say you build a barn out of concrete blocks. (The paint on steel siding does burn, the steel does not.) Only latex paint can be put on the walls. I'm not sure what you can use for a roof that doesn't burn at all. It's usually to tell after the fact because the heat is great enough to cause warping, but let's say the roof is fireproof.
Now let's look at the fire load. This is actually the biggest part of the equation. Fire load consists of the structure and everything in it. A piano is part of the fire load and burn very nicely. In a barn, all of the hay is an incredible fire load. That's why many farmers keep it in a separate structure. But all that hay leave dust and clippings, all flammable.
Leather tack, when it gets hot enough, burns. Oil and gas (including the drippings from a tractor) burn. The cover for electric wire burns. The plastic feed dish burns. Lead ropes burn. Grain burns. Should I go on? Bottom line is a barn has an incredible amount of fire load.
Then you have to look at design. Small rooms that have the door closed use up the oxygen in the room quickly, and then smolder. Unless they can get more oxygen, the fire will go out. Large open spaces burn quickly and easily because there's a lot of oxygen.
And with a stone or metal walls, the fire is contained. This means the heat can't dissipate. As a result, stone or metal walls will actually increase the temperature of the fire.
Net result is that barns work by keeping sources of ignition out, like no smoking and no kids that burst into flames.
Best of luck,
Jim Clark-Dawe