target speed

What make/model radar guns are you using to measure the speed of the targets? I'm looking for opinions/experience/recommendations of what to purchase for our club. We found one on Amazon but don't know if it's a good one or not: Amazon.com : Bushnell Velocity Speed Gun : Radar Detectors ...

I've been playing around with a spreadsheet that compares leads/shot velocity/drops etc. The one piece I can't find is how much and how fast the clay slows down once it leaves the trap. Does anyone have actual data or educated guess on the speed vs distance of a clay target from a trap...