When using a chronograph with centerfire ammo, you have only the bullet going across the skyscreens so no false readings. But, when you are shooting a patched round ball, you can get false readings from the patch if it crosses the skyscreen. What you have to do is to see where your patches are landing and set your chronograph just a bit beyond that point. That way, the patch will not interfere with the reading of the velocity. Most of the chronograph software has a place where you enter the distance of the chronograph from the muzzle. It will automatically make the corrections necessary to give the muzzle velocity. If you are shooting conical bullets with sabots, you will need to do the same thing. I have never chronographed a shotgun load so I am not sure how to do it. If you set the chronograph just beyond where the wads are landing, the shot pattern will be so large that it will hit the metal rods that hold up the plastic screens. I'm sure there is a way to do it but I don't know what it is.