A simple mod I came up with while messing with my old nerf gun. Really what it does, is when you fire a dart, a dart will be shot, but it will have a puff of "smoke" (dart pieces) fly out behind it, giving it the effect of gun smoke.
And really all you need to do is cut a dart in half, and then cut up the rest into rings.
Here's a vid that shows the effect and a better tutorial: http://www.youtube.com/watch?v=tlzUtKRvQAs&feature=youtu.be
And don't forget to check out the other things I've made and sub for more!