I have been following the thread "Let's argue about BC's " and oh my head hurts! Anyway I was thinking about all of the other things that might make a bullet appear to follow a trajectory, different from that predicted by ballistic software. So I thought I would come up with a top 10 list and I hope that the smart Long Range Hunters here can add to it to come up with a definitive list on how to make a good rifle shoot bad. 1. The Shooter. 2. Scope calibration error 3. Chronographed velocity wrong. 4. Parallax. 5. wind. 6. Mirage. 7. Atmospheric readings wrong. 8. Cant. 9. Distance measurement error 10. Variations in rifle hold, rear bag placement, recoil control, cheek weld etc between different field shooting positions.