How much is the amount that need a storage locker?
Look on line for your state's fire code. You might also look for county and city ordinances regarding this. Since all the places I have lived in the U. S. allow the commercial sales of both black and smokeless power without specific legal warnings, that implies the legal right to keep it at home.
SAAMI has some advice on the matter. I build some boxes using 5/8 inch plywood doubled up which makes for over an inch of wall thickness. One is supposed to put no more than XX pounds (can't remember it all off the top of my head, but it amounts to a fair amount) in one box (pile). I don't have 'specs' but I didn't have much problem figuring out how high and wide it had to be to house several pound cans of powder. The bottom and sides are all double layered plywood (as above) and the top is the same with a raised lining to press fit the top in place. Keep the immediate flames out and allow the pressure to vent if it catches fire.
Since I only have smokeless powder, I am more concerned with primers.
Primers can be set off by heat or impact and tend to chain fire. The proper term for this is 'sympathetic explosion'. If one goes off, the one next to it goes off. So fast it looks like one explosion.
Smokeless powders will burn and ignite rather easy - compared to logs, anyway. Probably easier than single sheets of paper. However the burning rate when NOT pressurized (like it is in a cartridge) is a rapid evolution of smoke and stuff, but not an explosion. Probably less scary than gasoline. Still gets one heart rate up and will probably require a 'change of linen'. But it's over in a few seconds.
I found some information a couple years ago on line under 'universal fire code' but the current finding doesn't mention 'gunpowder'. Better to search on your own than to blindly take my word.
I never really understood this steel case with a "blow out" plate. Ahh why. If the plate does blow like it should how is that different from it sitting on a shelf? Answer it really is not short of just holding it in a specific area.
It's a measure to protect fire fighters. If you have 64 pounds of powder locked in a safe that's no gooder.I must admit... I've scratched my head over the whole 'powder magazine' thing... a storage box that falls apart. I understand the concept of why the box needs to relieve pressure, but I question the need for a box at all. If a reloading room is engulfed in fire, having the powder in a box that will come apart seems senseless to me. Certainly I'm missing something...
Probably the single best concept explanation I've seen. The wood box is to separate the contents from flame. The thickness is to insulate the contents from heat. The 'blowout' panel or 'flimsy' construction is to prevent the box from becoming a bomb in the worst possible eventuality.The WOODEN box is supposed to provide a fair amount of insulation and consumes energy by charring. So if the fire is not too long lasting, you might not find out about the blowout panel or loose lid.
Probably the single best concept explanation I've seen. The wood box is to separate the contents from flame. The thickness is to insulate the contents from heat. The 'blowout' panel or 'flimsy' construction is to prevent the box from becoming a bomb in the worst possible eventuality.
To be open, I have a suspicion that storing the powder in a small room is probably not too dangerous. And frankly, smokeless powder is less of a hazard than some cleaning solutions. A gasoline lawnmower with the fuel tank half full has far more explosive power than an eight pound keg of smokeless powder.