Do tell...
Type: Posts; User: Magnetite
Do tell...
It could be not enough shots to find the true centre. Hornady (podcast ep. 50) did some testing and found five shot groups averaged +/-0.3" away from the true centre at 100yd. A chronograph will let...
Did you confirm zero at the same time you shot 400 and 500?
Did you shoot 400 and 500 on the same day?
Did you account for aerodynamic jump?
How many shots did you use to zero, at 400, and at 500?