It is really not hard to add a few layers onto your scene to create great looking gun fire. And this video will show you how
To create this effect, you will have to composite a number of layers. Of course, these are not hard set rules so experiment if something doesn't seem to fit right - to me, that is always the most fun part anyways
Anyways, take and position a muzzle flash element on top of your base footage.
Next, add another solid colour layer to simulate the light emitting from the muzzle flash. Set it to a colour that is very similar to the muzzle flash and change the blend mode to additive. I usually set the opacity to around 20.
Just putting a solid layer over everything does look pretty crappy. If there was a real muzzle flash in the scene, only the elements facing the front of the gun would be lit. We can simulate this by applying some masks and cutting out only the areas that would directly be hit by the light.
Once you're done, your shot should look something like this: