very interesting that all your chrono needed was to watch for a drop in light. I'm wondering how far above the sensors the bullet can be as still be detected.
The window is a triangle about a foot or 18" tall.
I just visited the manufacturer's web site (not much hard data there), looks like some of their newer models include IR illuminators, so it's watching for a reflection rather than a shadow. The older passive models are still available.
-j