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Why are the culverts there? Noise abatement?
Safety. That way, people cannot shoot OVER the backstop
My problem was vertical. I had it sitting on the shooting bench while I was standing and it missed the first two shots I took. I then took a wooden block about 18" long, set the block vertically, and then put it on that block - read everything after that.
The reason for accurate placement in relation to the muzzle (this is for all RADAR based chronographs) is because the actual muzzle velocity must be back calculated. The bullet is picked up X amount of distance after it is fired. Even on our large radars we need to accurately measure the x, y, z axis the muzzle sits from the trigger and radar. This is important because the bullet starts to slow down immediately. (believe it or not even something "on the muzzle" like a MagnetoSpeed isn't immune to this but the distance makes it irrelevant). Placing the Garmin and other Radar Chronographs as precisely you can to where the device thinks it is helps improve the accuracy of the reading. These devices use that distance to back calculate the actual muzzle velocity vs the measured velocity. You can see this in the LabRadars** by looking at the log files and you will see that the measured velocity and the reported muzzle velocity are slightly different by a couple FPS.
