Thanks for the help guys.
The application is a reactive target for pistol matches. I read the various threads dealing with detecting the passage of a bullet within a specific scoring area and it doesn't appear that anyone came up with a solution.
In our matches, we use cardboard targets. My idea is to have a 6"X12" card stock tapped to the top of the cardboard target and hanging down the back. The card stock will be covering the scoring area of the cardboard target. When a bullet passes through the cardboard target in the scoring area, the card stock will swing backwards and this is the movement I need the Arduino to detect and trigger a relay.
The sensor needs to be mounted at the bottom of the target stand to make it less likely to get shot. If it would help, I could mount PVC pipe over the sensor to cut down on the area it would be "seeing," if the pipe wouldn't trigger the sensor.
(I mentioned white card stock, but it can be any color if that would help. It could even be tan, black or any other color cardboard if that would be better.)
Does my design sound feasible, or do I need to start over? (We have all of the mechanical aspects worked out and tested, including accounting for wind blowing the card stock.)
Thanks again guys,