The recommendations are to not store it in a safe. The assumption is the safe is airtight or close to airtight and if something lights off the expanding gas will have no place to go and then you get a really serious explosion.
There is plenty of BATF regulations on exactly what is legal.
For example, you are not supposed to store your powder and primers in same location.
I finally broke down and got a plastic box from Plano. Academy has a similar one on sale.
I keep black powder in one box. a fair distance away is powder. Primers are close but far enough from both.
I can tell you never ever have more than one pound of powder on your bench in use. During reload is when you can have a primer light off and having three or 4 go off at once is a sight to behold. I believe they could easy burn through the plastic of most powder containers and once you got one, you will have more.
Several years back I had some smokeless I wanted to get rid of and teach my kids a lesson about safety. I poured about 1/2 pound in a 6 foot line and standing about 3 foot away dropped a lit kitchen match on the small end. Flames reached 8 foot tall, easy. And the heat was significant.