You'll be lucky!
I have some s/w (ParticleIllusion 3) that does a reasonable job but it's not free.
The demo leaves a watermark over the final render but as muzzle flashes tend not to fit the whole screen you might be able to position the thing just right so you can generate some that are usable. Pop over to
www.wondertouch.com and have a look.
But to be honest, I think your best bet is to find a muzzle flash image/clip (the best one I have came on a royalty free clip disk from Artbeats [I think]). Take the flash image and simply place it on your clip for one or maybe two frames, resizing and rotating it to requirements.
This is how I've always done it and no-one has told me how tacky they look yet so they must be ok.