Locking up ammo has NOTHING TO DO WITH FIRE SAFETY!
If it was about fire safety, almost all cleaning chemicals, hair spray, etc. would have to be stored in locked wooden boxes!
My Fire Chief (a good guy) admitted that those regs re locked storage are NOT related to true fire safety. It is obvious that one of the HCI gang snuck in the back door and bent the ear of the State Fire Marshal at some time while we were watching the front door (legislation being filed).
The classification of ammo as explosives is no doubt a DOT (now Homeland Security) invention, as they create and define what is what and how it has to be labeled and shipped interstate.
I'm sure that the national fire chiefs' organization probably promulgated suggested regs on powder/ammo/primer storage permits and most states probably have something similar on the books.