Are you looking for how to create the muzzle flash or just how to composite an existing bit of 'flash' footage on top of another clip?
If you have the flash footage (actually I tend to use single frame stills) already then you don't need AE6.5 at all. I do this in Premiere Pro as follows.
Import muzzle flash still/footage plus video clip of gun.
Place gun clip on layer one.
Place flash clip/still on layer two (on top of the gun clip).
Resize flash clip. this si ok as long as you are just reducing the size and not zooming.
Rotate and position flash on end of gun barrel.
This is exactly how I did the muzzle flashes in Final Hit (
http://www.alandmills.co.uk/Final_Hit.wmv ). See if it works for you.
If you want to create the actual muzzle flash then that is another story. ParticleIllusion is good for this job but it's not cheap if that's all you want of it. Your best bet would be to find some stock footage.
hope this helps.