My personal choice is the Pact Club Timer. You can get one thru Dillon's, Denney's Shooter's Supply or your local dealer. I paid about $125 for mine.
The timer works by recording the time for each shot fired to the nearest 100th of a second. The concussion of the muzzle blast triggers the timer. They come with instructions but you really should join a club near you. You can save a lot of time and money that way.