Do not store blackpowder in a steel ammo can. The tougher the container the bigger the BOOM!
If your going to store it in a metal box make sure there is a "path of least resistance" such as a non locking lid or very thin soft sheet metal.
I agree with this; storage in an ammo can is usually safe, as long as other precations are taken, such a baracading the can in such a way that if it detonates, its forced against an outside wall. Most local areas have city ordinances that allow only 50lbs max to be stored in a home. I never keep that much around, because I've personally detonated 10lbs of black powder at an outdoor demo range. Secured inside a home, that much depending on location could take out a 2000+ square foot home. Its the reason I listed it in my original post and how to store it. The biggest problem with black powder isn't storage as much as temp change, as it tends to draw mosture to it. Make sure where ever you have it stored put some dessicant in with it.