Ok let me give this a whirl. For FREE ideas use maybe a lens flare or other light directing effects instead of shine, but if you have or can get shine it goes like this:

If you have or can get the trapcode plugins "shine" or "Star glow" these will make a great half second flash effect when shown through a masked layer. An idea (not tested but it works in my head) EDIT: Tested and works see below for a quick muzzle flash effect I made as an example.

1.) Create a new title using the titler.
2.) Inside the title place a small elipse/circle (just enough to cover half of your "muzzle" any color except black).
3.) Save the title then place it in a video track over the area you need the effect. Use your motion option to place the elipse where you need it.
4.) Drag starglow or shine into your title. I'd use shine even though most use it for longer animations.
5.) Make the source point just behind the elipse so the rays face the direction you want the flash to go.
6.) on the time line reduce the title to a fraction of a second.
7.) Choose: one color or 3 color gradient (warm heaven is a great preset).
8.) keyframe the boost light option at the "begining" of your flash to 0, do the same for the source opacity.
9.) At it's "peak/fire" set opacity to 100 and boost light till the shine shows a good flash.
10.) at the end reduce the boost light and source opacity to 0.

For multiple muzzle flashes just recreate as many titles as you need copy the original and use the "paste attributes" function. Now I'm going to go test this out to see my brain is telling the truth or not.

EDIT: It works! Uploading the file now. Total time to create= 5 minutes. Once you do the first flash it's just a matter of copying and pasting. Just copy the title frames and paste wherever you need then use the motion option to place it where you need.

File: Muzzle flash test