kyreloader;
My first instinct for an explanation is that it is a paralax issue, but I am perfectly willing to accept that it is properly adjusted.
I am perfectly willing to accept your group results.
What I am unable to accept is the explanation for the phenomenon. I cannot believe that a bullet can accurately and reliably turn an angle at some point in its flight.
As for Gyroscopic precession, it is about a gyroscope with a gimbal returning to it's natural axis after a disruption or initial instability. My issue with this explanation is that a bullet has no gimbal. Precession explains a bullet becoming stable, but does not explain it turning an angle in order to converge at an aiming point. http://en.wikipedia.org/wiki/Gyroscopic_precession#Torque-induced_precession

All this is to say that I believe the experience of the posters, I just don't know the explanation.

Democracy is not freedom. Democracy is two wolves and a lamb voting on what to eat for lunch. Freedom comes from the recognition of certain rights which may not be taken, not even by a 99% vote.
*Marvin Simkin* L.A. Times (1992)