Hmmm. It's theoretically not that hard to create gunpowder, however depending on the purpose it is quiet possible that the stuff from a fire arms store (primarily for recharging spent cartridges) might not be the best for fire works (based upon different consistencies of basic ingredients). Ah, not that I have tried before 😗
This site though gives some basics
http://chemistry.about.com/cs/howtos/ht/gunpowder.htm
Note - this site is primarily for general knowledge.
finely divided carbon (soot) plus salt peter (potassium manganate) plu sulphur powder. carbon is the fuel, salt peter provides the oxygen and sulphur gives the spark. mix em REAL good, but NOT in a metallic container with a metal spoon or sumthin. gunpowder! muck aorund with the ratio of quantities to get a faster or slower burn...add iron and aluminium filings to make it extra hot...