I am an estimator/designer for a fire protection contractor. Halon has not been legal for civilian use for many years now. Currently the chemical suppression system that are in use are FM200 or Novec 1230. The cost of these systems can be rather expensive and far greater than I would want to spend for powder storage.
Kidde Fire Systems: High-End Residential
If you are concerned, you may want to look into a rated fire cabinet. I think this would be your best bet.
Flammable Storage Cabinets at GLOBALindustrial.com
You could do a small automatic fire sprinkler system tied into your domestic water supply. If you tie into the domestic water just make sure you have a backflow prevention device to keep the stagnate water from getting into the homes regular drinking water and some type of local area alarm to notify you of water flow.
The truth of the matter is that a fire in an area where there are stores of gun powder would be fueled so quickly that a fire sprinkler system would probably not do much. In military and manufacturing setting they would use a system that employees sensors that would detect a potential fire or explosion before it starts. This is definitely over kill and would be extreme expensive.
Here is a link below from the last NFPA conference regarding powder, bullets...etc.
Bullets, gun powder, and primers--oh my! NFPA staff discuss ammunition's reaction to fire - NFPA Conference & Expo