"ProChrono" by Competion Electronics
They also make timers and other electronic units for shooters. Easy to use, runs on a 9v battery. While gases have blown off the front plastic face cover, it easily goes back in place, and I haven't put a bullet through it yet. Knock on Wood.
For reloaders, especially rifle reloaders, I agree with ak4wd that their a necessity. I have a ballistic program in my old PDA, which I take to the range with me. Once I know the velocity, which can be different than the manuals; the ballistics program helps me sight in my rifle to maximize that load based on that velocity.