The chemicals used in these devices are relatively stable, if stored under the right conditions of light, temperature and humidity. Also please note that typically these pyrotechnics are packaged in individual sealed metal containers, inside sealed waterproof wrappings. However, they do represent serious fire hazards, due, particularly, to the white phosphorus which is a basic ingredient of many of these items. This is why the military tends to segregate these from other stored items, often keeping them in separate "dumps". I suggest treating it as ammunition. With the exception of a small quantity of blank ammunition and black powder used in re-enactments, we store all such materials off-site, at a licensed range's storage facility; if I had any pyrotechnics (which I don't), I'd store them there as well. Harry Needham Canadian War Museum