In case of a fire in the area where the ammo boxes are stored, wouldn't a couple thousand primers in a tightly sealed container be a bomb? I would think a "cook-off" would be disastrous in this case...
A fire in an area of several thousand loaded cartridges in a G.I. ammo box would then also be problematic. But, the military still does it. I choose not to worry about a fire and my primers, I worry about a meteorite striking me on the head when I am out mushrooming...you have to choose your "worries".