Just because I have, say five loads that produce one inch groups at a hundred yards, it doesn't mean that all will produce the theoretical five inch groups at five hundred. One will be better than the rest and may shoot better than five inches, and one worse than the rest, the others in the middle.
Bullet quality can definitely be a factor but the nut behind the wheel must be on top of his game not just to do the shooting but also to be able to recognize the many and varied issues involved.
If this is something that was able to be accomplished in the past at 500 then it is somewhat different than if it has never been done to date. That said, I agree that the wild velocity changes are not helping consistent barrel harmonics.
