Spin Drift/Magnus effect - What is it?
A variable that isn't well understood by most, but here is a youtube video showing the effect on a basketball falling with a backspin.
As you can see the effect is not only real, but have quite a profound effect on the trajectory. The reason it does this, is due to pressure differential created by the spin itself. On one side the surface moves with the airflow, and on the opposite side it moves against the airflow direction. This creates a difference in pressure, and as rudimentary high school science knowledge have taught us, things wants to move from high pressure towards low pressure. There is also airflow separation on the side that spins against the airflow direction, but that's getting into technical mojo...
The video shows an example of how the Magnus effect will push a projectile of the vertical centerline when affected by a cross wind, the direction is dependent on twist direction. In the video I've used the common clockwise direction. (you can loop the video by right clicking the play button & select loop)
You would think the POI (Point Of Impact) would be located on a eclipse, but it seems to be more like a skewed box with rounded of corners. This would be due to the shape of the diabolo pellet, and it's low BC (Ballistic Coefficient) Which is relatively low 0.015 to 0.030. The higher this number is, the flatter the and rounder the eclipse becomes. But one very interesting detail is that the most favorable wind conditions for shooters would be wind from 10:30 and 4:30 where the magnus effect is essentially gone.
The amount of drift is obviously a result of various factors, such as the velocity of the projectile, the spin rate of the projectile, Bc, and the velocity of the wind. So the above example isn't showing a solution to any and all wind conditions. But it shows that the magnus effect and wind means that the maximum wind drift of center bull, is at clean sidewind but is of the horizontal centerline depending on from which direction the wind comes. If it comes from the right, you'll be under the line, if it's from the left it'll be above the line.
in target shooting you need to know what spin direction the projectile have, i.e. the rifling in your barrel, clockwise or counterclockwise. Most barrels I have encountered are clockwise, and for the rest of this post, I am assuming a (clockwise) right twist. This means that wind coming from the right will create a increased drop of the projectile, more than it would in a no wind condition. If it comes from the left, it will rise a little bit, flattening out the trajectory a little.
If you been shooting in windy conditions at ranges with structures around it, buildings, trees etc. You would most likely also have seen up/down drafts, which also put a magnus effect on the projectile. A down draft would make the projectile drop, but because of the magnus effect it will also drift to right. If there is an updraft caused by ground undulations, or turbulence, it would induce a drift up and to the left.
Clean side wind condition
So in short, you need to account/expect a little upwards deflection when the wind is from the left, downward deflection when the wind is coming in from the right. The caviat is the projectile shape and length, in some projectiles this will be reversed. What exactly causes it, I dont know. But if you are shooting a pellet of regular shape (diablo) It most likely going to behave as outlined.
Since up/down drafts are usually gusty and localized, those are probably the most difficult to predict. Probably best accounted by learning the particular range you are shooting at to see what the conditions are. In most cases down drafts happens either in head wind at the far end of the range, caused by the berm. Or in tail wind when you sitting under a structure and your target is close, sub 50m.
Updrafts are most common in the middle of the range in either tail or head wind if there are sharp berms running perpendicular to the shooting trajectory.
If you have very high side berms and your shooting position is close to either of those sides. When you have side wind it will cause turbulence in those areas. Rule of thumb is that if you are on the right side of the range, with a tall berm on the side. If the wind then is coming in from the right, you will get a downdraft with a left moving turbulent wind, meaning that you will have a wind drift to the left, and a magnus effect causing drift to the right, and slightly down. if you have a clockwise twist barrel, you are better of on the far right side of the range, than the far left side. Since the magnus effect caused by any up/downdraft will negate some of the drift caused by the primary wind direction.
If the wind is coming in from the left, you could experience updrafts as the wind wants to go over the berm, but it creates a fair bit of turbulence since the wind will create a high pressure area in front of the berm. This pressure is not stable so the curve the wind takes over the berm will pulse left/right/front/rear causing the the drift to be unstable. As well as causing additional left and up drift by the magnus effect.
If you sit on the other side, the opposite if true, wind from the left wind will cause a down draft at this location causing the a bit of right & up drift by the magnus effect.
Obviously in windy conditions, you are better of where the wind is as stable as possible, which would be slightly to the right of center in right wind conditions, and slightly to left of center in left wind conditions.