We did coriolis to death a while back; can't summon up the will to rehash it here so here's a link:
Drift (or spindrift) is one of the few 'weird and wonderful' ballistic effects observable and measurable at 1000yds...the others (coriolis etc) are all at play, but not measurable. Drift absolutely is.
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Itís not yaw, but precession caused by the bulletís tip continuously pointing down as its spin axis stays parallel to the trajectory path. As the bullet noses over, its right-hand twist causes it to change its direction to, or drift to the right.
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...actually, projectiles fly slightly nose up in relation to their trajectory; this means that air resistance will try to lift the projectile's nose. Because the projectile is a powerfully spinning gyroscope it reacts to this upward pressure by yawing slightly to the right (it's just something gyroscopes do), with this new yaw, the changed direction of air resistance causes the nose to move down (again, because of the weird way gyroscopes react to forces)...then it will move left...then it will move up ....and so on round and round.
This creates an effect known as 'nutational rotation' ..a more complex movement than I can describe without drawing a picture.
....anyway, the net effect of all this movement is that (clockwise spun) projectiles average a 'nose high and right' attitude in relation to the line of their trajectory. This 'average orientation' of the projectile known as the yaw of repose. ...it should be obvious why a 'high and right' yaw of repose (also known as 'equilibrium yaw') will cause increased air pressure on the left side of the projectile and thus cause it to drift to the right.
...I think you'll find the values quoted by JBM are the average accepted 'norms'.....take a peek through a direct fire antitank gun sight next time you have a chance; their reticles are actually marked to account for drift...even at relatively short range [img]/ubbthreads/images/graemlins/smile.gif[/img].