I store mine in an old refrigerator, powder in the fridge, primers in the freezer. The magnetic seal on the door will open under moderate pressure and allow the gases to escape in the event of a fire. The insulation in the fridge helps to moderate the temp swings as my reloading room is not climate controlled.
Here is what I found on the ATF website regarding storage of smokeless powder:
Is smokeless powder designed for use in small arms ammunition subject to the explosives storage requirements?
Smokeless powders designed for use in small arms ammunition are exempt from regulation under 18 U.S.C. Chapter 40 and the regulations in 27 CFR Part 555. Packaging that readily identifies the smokeless powder as being designed for use in small arms ammunition may help in determining whether it is entitled to the exemption. Smokeless powder designed for use other than in small arms ammunition, and explosive products such as squibs, fireworks, theatrical special effects, or other articles that may contain smokeless powders, are regulated and must be stored pursuant to the regulations at 27 CFR 555, Subpart K – Storage.
It should be noted that persons engaged in the business of importing or manufacturing smokeless powder designed for any use must have a Federal explosives license. Further, importers of smokeless powder designed for use in small arms ammunition must also possess an ATF firearms importers license (Type 08 or 11); must register with ATF under the provisions of the Arms Export Control Act; and must submit (to ATF) and receive an approved ATF Form 6 – part I (5330.3A), Application and Permit for Importation of Firearms Ammunition and Implements of War.
What are the classes of explosive materials for storage purposes?
There are 3 classes of explosive materials:
(a) High explosives (for example, dynamite, flash powders, and bulk salutes);
(b) Low explosives (for example, black powder, safety fuses, igniters, igniter cords, fuse lighters, and “display fireworks”, except for bulk salutes); and
(c) Blasting agents (for example, ammonium nitrate-fuel oil and certain water gels).
[27 CFR 555.202]
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You will note that smokeless gun powder is not listed above as an explosive and is exempt from storage regulations when used for small arms ammunition.