Black Powder, and your smokless powder stored in its cans are the biggest worry. Loaded ammo burning and blowing isn't that big of an issue even in extreamly large quantities. For most small arms ammunition the powder and primer will burn insde the casing, and since all explosive material seeks the path of least resistance, it will push the projectile out of the caseing. Since there is no barrel, or rifleing to control the projectile, it will only go a few feet from the round.
Good stuff, I have some really old Black Powder I need to move and put in a metal container in the garage. Thanks for the good info.