You need to know what your actual velocity is as a monitor on pressure and to calculate your down range ballistics. You also need to know how consistent your velocity is. While shot to shot variations in velocity may not affect you much up close, they cause vertical spread in your groups as the range increases.
HISTORY OF CHRONOGRAPHS
Most early chronographs used a series of sky screens that relied on seeing the shadow of the bullet passing over head to trigger a very accurate stopwatch that could then calculate the velocity from the time the projectile took to travel from the start screen to the stop screen.
These were plagued with problems such as varying sun/light angles and intensity, reflections off shiny bullets etc. To get around this on our home range we manufactured light tunnels which shielded the skyscreens from any ambient light and then provided consistentwell but was far from portable nor was it cheap! The first major leap forward in chronograph technology was Doppler radar and the Magneto speed. The latter unit was attached to the barrel near the muzzle and used electromagnetic sensors to “see” the projectile passing overhead. The issue with this was it affected the harmonics of your barrel so you couldn't shoot for accuracy at the same time as measuring velocity. Doppler radar (think of it as similar to the K band radar used by the police for detecting speeding drivers) was used by the military for some years before the first commercially available unit for civilian use became available – the LabRadar. This was a break through and finally provided a chronograph that would work in any light conditions while you were testing for accuracy and was very portable. Some people had real issues getting them to trigger reliably, as they relied on the sound of the shot to start the radar – which proved an issue with suppressors, air rifles and bows etc. I added the accessory microphone to ours which solved that problem and it became almost 100% reliable. While portable, it was still a reasonable sized unit and was quite heavy on battery usage. It also had the annoying habit of the “armed, ready to record” mode timing out just as you were ready to fire another shot, resulting in no velocity. It also required a little thought in how to set it up to avoid muzzle blast and to be able to record consistent velocities. Despite these short comings it has served us well as our field chronograph anytime we were away from our home range with its Oehler 43 ballistic laboratory light box setup.
