I bought this one a few years ago
http://www.midwayusa.com/product/531741/shooting-chrony-f1-chronograph
It works great but have to keep a mindful eye on the lighting conditions. Seems to work best with either a overcast or sun directly overhead the unit. Sometimes I have to point the top of the chrony toward the sun. I have been thinking a single large piece of opaque plex will fix the lighting issues. When the lighting isn't right the readings will either be way off or not read at all.
If I had to do it again I would get this unit.
http://www.midwayusa.com/product/628777/magnetospeed-v3-ballistic-chronograph?cm_vc=ProductFinding
I have used it and was impressed with it's accuracy of readings and no more lighting issues. Plus it can be used in any conditions. Sure it cost alot more, but I think it is well worth it. Only drawback is you cannot shoot for accuracy shots at the same time. Which having the velocity during load development is helpful to have another sign for pressure.
Maybe someday I will upgrade to this unit, but funds most of time go towards components. Plus it is hard to justify when my F1 works fine if I do my part and watch the lighting. Plus the only time I chrony my loads is after load development and need to get a idea how fast I am.