This is an often seen/debated topic on reloading forums. Mostly post by the "What if Brigade". I agree that a metal cabinet is good enouth as powder just burns outside of a confined space (cartridge case, ammo can, tightly closed small cabinet, etc. In a fire hot enough, a plastic powder container would melt and the powder burn, primers would pop like pop corn and loaded rounds will pop, spliting the case and not much else. Mostly not going to level the neighborhood or kill every firefighter in a 50 yard radius.Just like everything else reloading, think. Use common sense (My powder and primers live in two different cabinets. I don't have a 100 watt light bulb in my reloading cabinets and I don't store my components near a heat source (fireplace, wood stove room heater or when I would use my lead pot and/or hot plate). I would worry much more about my propane cylinders exploding in a fire (I have a couple in my SHTF supplies and one or two for smelting). In 74 years I have never had a house/garage/shed fire.
But I have seen quite a few reloaders post about their wooden powder storage "trunks" or cabinets, so if you wanna, go ahead, but think (I wouldn't store in a root cellar because of humidity/dampness, or place the trunk/cabinet next to a water heater or heater)...