I found this just messing around shooting the walls.
You have to shoot the walls about at the angle that i
am standing at. As far as i know it will only work on
Metal surfaces that will leave bullet holes in it. I
tried shooting snow and rock walls a few times and i
don't think it will work.
This is almost certainly the same effect that can be seen in the Blood Vent movie and the Green Smoke movie... but it's a pretty cool little visual glitch. :)