Shooting vs. Ballistics Cal.

[acourvil]

If you are using a budget chrony, you can easily have 100fps or more variation between the reading and the actual, but 300fps seems unlikely to be all data error. However, a combination of having your zero a little off, data error (both fps and BC) and failure to account for environmental changes could get you there. And of course, it's certainly possible that part of the variation is your technique, e.g., if you zeroed in one position with a stable rest, your field shooting position could be causing part of the vertical difference.