> If there was an initial set of fires and smaller explosions that dispersed unburned gunpowder and wood/grain dust into the sky, once it was dispersed a flash

> If there was an initial set of fires and smaller explosions that dispersed unburned gunpowder and wood/grain dust into the sky, once it was dispersed a flash could have spread through the dust cloud and created a huge explosion as described. That was my thought too, especially with the description of the initial rumbling noises. Gunpowder burning in a small space might well provide the energy needed to disperse something flammable in a cloud above the armory which could then be ignited at some interval after it was created, providing a fuel-air explosion above ground level which would explain the over pressure damage without burning, the shape of the mushroom cloud, and the magnitude of the explosion.