Effects of cross wind on elevation

[RockyMtnMT]

I know there was a thread on this not long ago, but I was not able to find it by a search.

Did a little shooting yesterday at 1035y with about a 20mph wind from 9 o'clock. Along with that the shot was across a deep canyon running steep downhill from left to right. So the wind was coming down from left to right. My elevation solution wound up being almost 2 moa more than I was expecting. We have seen variations in bc from one rife to the next with a given bullet, but this one was more. Bullet flight in this shooting location is about 1000' above the ground always making it difficult to read what the wind is doing.

Let me know your thoughts or if anyone knows the previous thread on the subject post the link.

 

[bigngreen]

Just ran what I had loaded, 147 eldm at 2885 at my elevation showed +3/4 moa needed from static zero wind speed to a 20 mph 9 o'clock. From a 3 o'clock 20 mph to a 9 o'clock +1.5.

 

[lancetkenyon]

For info on where POI will differ from POA depending on wind direction and the effect is has on the bullet.

 

[Creedmoor shooter]

I get various elevation changes where I shoot if the wind is gusty. I shoot over a couple valleys and I believe its caused by the wind gusts coming up out of the valleys that changes my elevation. I've had as much as 2moa change in the same outing depending on the wind gusts. Not sure if that applies here or not.

 

[ShtrRdy]

Along with possible up drafts from the terrain I think there are two other contributors. Aerodynamic Jump and Magnus Effect. I'm going off of what I've read in Bryan Litz's book "Applied Ballistics for Long Range Shooting".

 

[Hand Skills]

For those who have trouble interpreting Lance's Sundial, it's a chart for calculating spin drift.

Spin drift can really mess with your day. It is why some competitors bring both a left twist and a right twist to a match, and select one based on which way the wind is blowing that day.

If your bullet is spinning 'into' the wind the wind will actually provide some lift. (Number 3 in the chart)

If your bullet is spinning 'with' the wind, more airflow over top of the bullet results in higher pressure above the bullet so it drops more. (Number 9 in the chart)

 

[ShtrRdy]

For those who have trouble interpreting Lance's Sundial, it's a chart for calculating spin drift.

Spin drift can really mess with your day. It is why some competitors bring both a left twist and a right twist to a match, and select one based on which way the wind is blowing that day.

If your bullet is spinning 'into' the wind the wind will actually provide some lift. (Number 3 in the chart)

If your bullet is spinning 'with' the wind, more airflow over top of the bullet results in higher pressure above the bullet so it drops more. (Number 9 in the chart)

I respectfully disagree that this behavior is Spin Drift. What you described is the Magnus Effect.
https://en.m.wikipedia.org/wiki/Magnus_effect

Here is a description of Spin Drift.
https://loadoutroom.com/thearmsguid...rnal-ballistics-spin-drift-13-theory-section/

 

[Hand Skills]

Good to know! I'm not sure why you disagree as Magnus effect is what CAUSES spin drift. I appreciate the clarification of terms however.

 

[Canhunter35]

I know there was a thread on this not long ago, but I was not able to find it by a search.

Did a little shooting yesterday at 1035y with about a 20mph wind from 9 o'clock. Along with that the shot was across a deep canyon running steep downhill from left to right. So the wind was coming down from left to right. My elevation solution wound up being almost 2 moa more than I was expecting. We have seen variations in bc from one rife to the next with a given bullet, but this one was more. Bullet flight in this shooting location is about 1000' above the ground always making it difficult to read what the wind is doing.

Let me know your thoughts or if anyone knows the previous thread on the subject post the link.

I was shooting 1030yds this morning with a 6-8mph wind from 8:00, I was .75-1 Moa low, no valley between. 20mph is difficult stuff at that range and with a steep downhill valley funnelling the wind, could surely cause the Moa drop. I wouldn't worry about ur bc in that kind of wind imo. Just too unpredictable

 

[aebhunter]

Good thread. I have a spot I shoot that consistently has a wind blowing pretty hard at 3 oclock. Usually between 15-20mph. The gong is at 720 yards and I almost always forget to take off about 3/4 Mil and my first round is usually about that high. I do have a right hand twist barrel, and it is a cross canyon shot as well. Magnus effect. Going to have to remember that one for my LR vocabulary.

 

[adam32]

The Hornady 4dof app has an excellent wind feature where it instantly changes the drop correction...pretty enlightening to see how much difference wind direction and speed can make on drops.

 

[Capt RB]

The Hornady 4dof app has an excellent wind feature where it instantly changes the drop correction...pretty enlightening to see how much difference wind direction and speed can make on drops.

+1 here. I've switched over to the 4dof just because it has been money when wind is factored in the solution.
Lance's sun dial is an accurate description however most won't recall it when they put the solution into the gun. The 4dof will do it automatically.
It's no different than having the bullet impact right side of target when conditions seem perfect. Most just write it off to unseen wind down range.
The best picture of what actual flight looks like is the DTR reticle. Here you can see the vertical stadia line is not plumb to the earth. THis accounts for the drift. The wind holds account for jump and push depending which direction it is blowing.

 

[Canhunter35]

The Hornady 4dof app has an excellent wind feature where it instantly changes the drop correction...pretty enlightening to see how much difference wind direction and speed can make on drops.

I downloaded the 4dof app and input my data from this morning, it is spot on with the wind direction. Matched up perfectly!

 

[Creedmoor shooter]

I downloaded the 4dof app and input my data from this morning, it is spot on with the wind direction. Matched up perfectly!

The 4dof is always spot on for me when I've got all my inputs correct. Best part is its FREE!