Big Jim,
Go to the ATA website, click on "About the ATA",,then click "View the rule book. You will see "Radar Guns and Chronographs" and "Rules for the use of Radar Guns and Chronographs
to set Target Speed".
I have yet to hear of a club that regretted buying a high power radar gun, like the Decatur Prospeed CR-1K that Bob Schultz sells (an advertiser here). I have watched clubs struggle, using a low power gun, that does not allow one to randomly read the speed from behind the 27 yard line as an event is going on. Traps can change speed, both up and down for a couple reasons. A radar gun purchase shows that a club is serious about setting good targets. Shooters remember where good targets are thrown.
Mash on the Bob Schultz link.