I did a load of muzzle flashes for a film i did at school. I had this file with about 40 muzzle variations, which i put in using AE *which is all well and good, if you have AE* I simply imported them, positioned them, set mode to screen, then positioned them again so they were on the end of the gun. I also made sure the flash was only 1 frame long, to give the impression of a quick flash. I found it very effective, and looks very very similar to the muzzle flashes in one of Alans films *i think it was there i saw the flashes...hmm*