I have an old army surprlus ammo can with rubber gasket seal. If there is a fire, the seal will likely melt or burn so the can will no longer be airtight which is good. When a round of ammo cooks off, it makes a loud noise but the bullet and brass spins any which way as the energy is undirected. You would not have rounds sending a projectile flying miles away. That steel box in my opinion should do fine in normal circumstances and if it is not airtight even better if we're worrying about bomb-like action. Just make sure the area in general is dry and cool. Under these conditions a wooden box would do about the same. If it is damp and/or the box gets very cold you will have condensation.