NFPA 495, which governs explosive storage, has no requirements for automatic sprinkler protection of ammunition storage except in Mercantile occupancies.
Your post states "mainly ammunition storage." Are other explosive or pyrotechnics stored in the magazine? If so, that storage is in violation of NFPA 495 and BATF&E regulations.
If your storage is limited to ammunition, you can protect it with automatic sprinkler protection. However, the system is not required by NFPA 495 or the 2003 International Fire Code.
Once upon a time, I designed a sprinkler system for bldgs. at an Army Ammo factory. Corps of Engineering Standards called for 0.50 gpm/sq.ft. over the entire area (deluge system). Turned out that the existing plant (circa 1940) could not even come close to having enough water or pressure required.
Also, the system was to be activated by UV sensors placed throughout the bldg., pointed at certain spots.
