I was @ the indoor range the other day and a guy standing beside me did the same thing as you.
He was pumped that he did, so he thought he would share it with the guys behind the counter of the range. The guys replied back "that's why there's black dots sir!".
Check this tube out. Let it run half way through. LOL
http://www.youtube.com/watch?v=Su3C_...eature=related