Flash powder is a zinc, aluminum, potassium perchlorate mix. It is a real explosive with a much higher velocity than BP. Very hot stuff. It also produces a huge white flash, it was used for photographs before the electronic flash. It is also used in fireworks and by magicians for stage shows. You do not want it anywhere near a firearm. No matter it is tightly controlled today.
5F sounds like null-b or at least similar.