For the skeptics;
If you container is not rated as a safe, but is the typical RSC box, not too much worry. As has been stated, these boxes have holes, loose doors, & can therefore vent, relieving pressure.
However, to assume that all safes are like your 'safe', is fuzzy thinking at best. True safes don't have holes in them, as it's exceedingly hard to meet the U.L. burglary and fire ratings with holes in the safe. Many good safes also have intumescent seals on the door that expand under heat, to keep the gasses of a fire out of the interior & help seal against the exterior temperature rise.
But, if worse did come to worst, and under extreme circumstances, the ammo in this sealed enviroment did cook off, I don't want to be anywhere near it. Normal housefire, the chances are very low that the interior would get hot enough to damage the interior papers, let alone cook off ammo. But, somebody lives next to the bulk plant, don't they? So 600,000 gallons of petroleum can't ignite? All that flaming liquid wouldn't dare come over to your place? Of course not, you'd sue!
I'm just pointing out worst case scenario. All the rest of you that want to chance burning ammo in there with your guns, go right ahead & store in the RSC. I'm not your insurance agent, I don't care. I will feel sorry for the poor defenseless guns though.
900F