all yardage group size

[JohnKSa]

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.

Except that we have evidence that it doesn't always push the bullet up.

For every 2 MOA sight movement in windage, you have to make a 1 MOA change in elevation. UP for going right, DOWN for going left.

I think that Mr. Vaughn should spend some time on the range to test his theory.

 

[Bart B.]

If it is not Magnus, I'll call it something else. It happens.

 

[Unclenick]

Except that we have evidence that it doesn't always push the bullet up.

Let me clarify what I was saying: The actual Magnus force acting on the bullet in the vertical plane is miniscule, and rechecking in Robert L. McCoy's Modern Exterior Ballistics, Magnus force mainly acts in the horizontal plane, anyway. Any vertical component is so small that even though it would push up, regardless of the direction of the wind (or lack thereof) it's of an order of magnitude that would produce a small fraction of an inch of elevation difference at 1000 yards. Really, really small. McCoy says that at the BRL precision free-flight spark range tests, they were never able to detect, much less measure, Magnus force effects on projectiles at all.

I think that Mr. Vaughn should spend some time on the range to test his theory.

If you read Vaughn's book you get instrumentation envy. Everything is rigorously tested.

I think you've mistakenly got the impression that because Vaughn shows Magnus force is not responsible for vertical wind drift that he is somehow suggesting that there is no vertical wind drift. That's not the case at all. He just shows it has a different cause than Magnus force.

Vaughn goes on to predict the angle off horizontal that wind-induced stringing will have at different gyroscopic stability factors. He includes a high contrast image of a 200 yard group fired in varying 3:00 wind from a 6 mm BR integral machine rest test bed gun that fires bugholes in no wind. It shows the predicted 17° angle off horizontal for that bullet's stability factor (about 1.6). He shows that a bullet with an average stability factor of 3 will result in about 1 click elevation change for two clicks windage change, as Bart suggested, and with the same up and down direction relationship he suggested (for a right hand twist barrel; it reverses with a left hand twist barrel). A bullet with an average stability factor of 1.4 will be about 1 click up for 4 clicks left.