How to create a light effect on weapon fire

I don't know how this effect is called... so I'll explain it on an example.

Imagine a soldier that fires a gun in a dark forest. You don't see him, nor the gun, but you see the light that appears in front of the gun when he fires. Example of it can be seen on http://smod.wdfiles.com/local--files/weapon:ar2/wpn_pic01_c.jpg (it's not in the darkness, but firing creates that effect).

What is the best way to create such an effect? Are there any publicly available examples?