Question: what is the difference between flash powder and gunpowder?
Answer: Gunpowder, unconfined, burns quite slowly with a "whoosh" - you have to confine it tightly to make it go bang. With flash powder, on the other hand, the reaction is faster than the speed of sound, so the pressure front confines the reaction and even a small amount - a gram or so - unconfined explodes with a bang.