STORING AMMUNITION IN YOUR SAFE
Hollywood has done a great job making us believe that when burned, ammunition will explode and fire as if it were in a firearm. Old westerns and even more modern shows depict bullets sbjected to fire shooting and whizzing by with the same velocities and characteristic damage as those fired from a gun.
Fortunately, this is not true. Bullet cartridges exposed to heat will indeed burst open, but usually not with any great velocity. The powder has to be strongly confined in order to generate those velocities and propel the bullet. Generally what happens is that the cases will burst open and bits of the brass case will fly around a few feet, but not with much force. The cartridges explode one at a time rather than simultaneously. The idea of one huge instantaneous explosion resulting from stored ammunition in a fire is very unlikely if not impossible. A serious accident could occur Hollywood-Style if a loaded firearm was subjected to a fire. In this case, the strongly confined powder would ignite sending the bullet out of the guns muzzle with the same energy as if it were fired by squeezing the trigger.
Smokeless powder used in small arms cartridges, unlike black powder, unless strongly confined, doesn't explode when heated. It burns much the same way plastics burn. Rather than an explosion, it burns with a quick hot flame. Smokeless powder in its original container will ignite and add heat to a fire but doesn't explode.
Black powder, on the other hand, is very explosive and will explode even if it's not confined! It should be stored in small quantities in its original container and great care must be taken to keep it away from any spark including static electricity.
So, in general, I would feel comfortable storing small arms ammo in a quality safe-especially a fire-rated safe as this would reduce the chance of any burning at all. I would, however, keep ammunition and firearms separated in different safes. This would prevent any damage to your firearms in the unlikely event that the cartridges did burst open and pieces of the case flew around.
Storing small quantities of black powder in a quality fire-rated safe seems to be a safer option than not doing so, but just keep in mind that if it does explode, it will be like a bomb going off inside your safe.
Thanks for your time,