The magnetospeed works well. I just don't like it hanging on my barrel, so I rigged this set up. You need a long bench and have to maintain proper alignment with the barrel.
DF
I finally broke down and bought a magneto. It works really well when you have to shoot at a public range with people next to you. No going down to readjust your sky screens or anything.
It does mess with POI and accuracy though. So now range sessions take one trip to determine velocity and another (without the device attached) for accuracy testing.