jones1899 said:
Despite the fact that a bit of graphite was added to gunpowder after WW1 still doesn't account for the "swarm of bees" it creates through the warehouse.

Well, according to Wikipedia, it's not entirely far fetched:

"It is also highly diamagnetic, thus it will float in mid-air above a strong magnet." (This refers to graphite, not gunpowder)

Of course, the "swarm"-effect is just a stylized and exaggerated version of this effect, and I agree that the whole magnetism-idea was actually too complex to be realized in the movie without compromises (sometimes the box is more magnetic than some other times), but it would've been impossible to include all effects the magnetism would have.

Having the junk roll on the floor, the gun-straps and dog-tags attracted to the crate and those other things that WERE in the movie and showed the magnetism were enough to sell the idea. It's Hollywood-logic, but it's enough.... just like the "leap of faith" in LC, which worked in its own realm.