Muzzle flashes done well can really add production value to your movie. I create the muzzle flashes in Adobe Photoshop frame-by-frame using the paintbrush tool.

Export your gun shot clip as a filmstrip file then import that into Photoshop. You’ll see the sequence frame-by-frame. I create a new layer over the filmstrip file and set the layer to “color burn.”

Then I just paint something that looks like a muzzle flash (to me, at least) over two consecutive frames. Sometimes I use the blur tool to further shape the muzzle flash.

Then save the filmstrip file and re-import it into Adobe Premiere, Final Cut Pro, Avid, Sony Vegas, or your NLE of choice.

Alternatively, you can create muzzle flashes in Adobe Premiere or After Effects by overlaying still images of muzzle flashes onto your video and then keying out the background.