John and Bart,
FYI, Vaughn, who was Head Aeroballistician for Sandia National labs and who co-authored an American Institute of Aerodynamics and Astronautics paper in 1973 called "A Magnus Theory", said that bullet vertical drift is not due to Magnus effect. In Rifle Accuracy Facts (Precision Shooting Pub, 2nd ed, 2000, pp 195-196) he summarizes:
"A lively discussion recently took place in "Precision Shooting" on how the vertical component of wind drift must be due to Magnus force. Since I am very familiar with Magnus effects {references his paper here} I wrote an article that appeared in the November 1994 issue of "Precision Shooting" explaining that Magnus force acts in the wrong direction and is much too small to cause the observed effect."
Vaughn doesn't go into detail in the book, but the reason Magnus force is not responsible is that wind doesn't blow over a bullet sideways, which is necessary to create the effect. Instead, precession constantly corrects the point of the bullet into the wind coming at it, so a side wind results in the bullet pointing into the net vector of that wind combined with the head wind caused by the bullet's velocity. A bullet at 2500 fps in a 3:00 20 mph side wind, will correct its point 0.67° to the right so the net wind it experiences is at the same angle to it that it would experience in still air. Drag being straight to the rear of the bullet and opposite the direction it points to, the drag now has that same vector angle off the still air trajectory, which is what pulls the bullet to the left (wind drift) in the 3:00 wind. It is not being blown that way by wind against its side.
The only error in all that is that precession reaches equilibrium with directing the point of the bullet into the oncoming air stream slightly off straight at the yaw of repose. This is a very small angle on the order of one moa in most instances, and it is this small pointing error that is responsible for spin drift by introducing a slight side drag. This drag is to the right for bullets from a right hand twist barrel, and to the a left for those from a left hand twist barrel. This yaw actually turns the side of the bullet slightly into the wind on the side that the bottom surface of the bullet is rotating toward. So the bottom of the bullet always has the faster speed into that tiny off-axis wind vector with the result that the small amount of Magnus force it introduces is always pushing the bullet up. That will be true whether the wind is from the left or from the right, and true whether the bullet is spinning clockwise or counterclockwise.
Vaughn goes on to say:
"Instead of Magnus effects causing the vertical wind drift component it is caused by gyroscopic moments similar to {those that cause} the yaw of repose…."