If all you care about is getting a maybe-accurate reading of the load's speed, use any hobby chrono. If you want a definitely-accurate report of the load's speed and chamber pressure, go to somebody who's set up to provide that info.
+1 for the ProChrono. Just remember to turn it off when done, as I don't think there's an auto off feature. Funny thing is, they include a spare battery compartment so you're not SOL the next time you take it out after you've left it on. Got mine from Midway.