If everything we did was prudent, we would none of us shoot cannons just for the fun it. At least in the eyes of some, it just isn't prudent to do so.
I would never advocate taking an unreasonable risk. I would point out however that BATFE's regulations are written based on the needs of commercial storage of explosives, not necessarily for the residential storage of powder by the average cannon shooter. For example, the requirement of five-tumbler locks is based on security of the magazine, not the safe storage of it's contents. The concern that prompted that regulation is the theft of large quantities of high explosives, and such a lock is not something that you just walk into your local Lowe's and pick up off the rack. From a safety standpoint, any high quality padlock with a case hardened shackle would probably serve just as well.
From a safety standpoint I'm much more concerned that the magazine be constructed or lined with non-sparking, non-conductive, fire-resistant materials and that it be located well away from any occupied building.
It is worth noting that BATFE regulations and standards are not intended for HOME storage of any explosive materials. That comes under the purview of your local fire code, which is far more restrictive.
