Wind and Ballistics

[JPG19]

I was just watching a show on the Military Channel titled "Modern Sniper: Army" and it had a sniper instructor on there discussing wind. To paraphrase him he said "Wind has the greatest effect on a bullet when it's halfway to or near a target." Is this true? I have no formal training, but was always under the impression that wind at the muzzle would have the greatest effect. Think: if the bullet is blown to the left 1 degree at the muzzle, that will translate into several feet at the target. If it is blown to the left 1 degree when it is 10m away from the target, it would be much less. Am I way off?

 

[taliv]

the debate continues (needlessly)

however, it's not precisely accurate. The mid-range wind doesn't have the greatest effect necessarily, but it is the spot most people look at wind.

1 factor in support of that statement is that winds are generally stronger the farther away from the ground you go, and the bullet is at the apex of its trajectory somewhere near halfway between the rifle and target.

the other considerations are:
as you noted, a lateral force at the muzzle changes the vector and since it is angular, it will have a greater effect than the same force applied near the target.

HOWEVER that assumes the velocity of the bullet remains constant, which of course it does not. A typical 308win fired to 1000 yards will be going around 2750 at the muzzle but around 1100 near the target (if you're lucky). So the force applied near the target acts on the projectile almost 3x as long.

make sense?

 

[JPG19]

It does, thanks for presenting the varying arguments. And for increasing my rapidly growing interest in long range shooting. If only there were a local locale for such shots, I could get outside and test these things for myself!

 

[taliv]

you're actually very centrally located to a number of great events, even though there may not be any "local".

MO, OK and western TN and northern MS all are home to great ranges and matches. And of course, the entire state of TX.

 

[allaroundhunter]

I have been taught to make sure to pay special attention to the 1st and the last 1/3rd of the flight path. As you said, the initial wind has the ability to cause the greatest angular change of the round. However, many people discard the final 1/3rd of wind because they figure that it won't affect the bullet much due to the bullet almost being to its target. As taliv pointed out, though, it is during this range that the bullet is at its slowest and is being subjected to wind forces for the greatest amount of time.


"When does the wind have the most effect on a bullet" is a loaded question if you ask me. The wind will literally push a bullet most when the bullet is slowest. However, if you consider 3 different shots - a 10 mph wind in the first 1/3rd of the flight path (for the 1st shot), a 10 mph wind in the second 1/3rd of the flight path (for the 2nd shot), and a 10 mph wind in the third 1/3rd of the flight path (for the third path) then you will see that the point of impact will be the greatest difference from the point of aim on your first shot. This, of course, given you are shooting at a great enough range to see the difference in the shots.

 

[JPG19]

Oh I agree, there are places within driving distance. In order to convince myself to bight the bullet (no pun intended) and buy a dedicated long-range rifle with optics, bipod, classes, etc., I'd want to be able to shoot/practice more often. I can take losing...I can't take getting spanked.

 

[45_auto]

The sniper instructor is correct as long as you're trying to keep things simple, with constant wind, at a "television viewer" level.

The slower the bullet moves, the more effect the wind has on it.

Think about it as if the bullet is trying to swim across a river with a fast current flowing. If the bullet moves fast, the current only carries it downstream a small amount before it reaches the opposite bank. If the bullet moves slowly, the current will carry it downstream a great distance before it reaches the opposite bank. The slower the bullet moves, the more effect the current has on it.

As a rough estimate you can use the "Rule of Squares". For instance, many people think that in a constant wind, if a bullet drifted 1" at 100 yards, it would drift 2" at 200 yards, 4" at 400 yrds, 6" at 600 yards, etc. In reality, you need to square the distance difference to approximate the wind drift. For instance, a bullet drifts 1" at 100 yards and you want to know how far it would drift at 600 yards. Since 600 is 6 times 100, you square the distance difference. 6x6 =36 so the bullet will drift about 36 times the distance it did at 100 yards, or 36".

If the bullet drifted 5" at 300 yards, it would drift about 20" at 600 yards. 600 is 2 x 300, 2x2=4, 4x5"=20"

You can see the difference in this wind drift chart for a 6mm BR:



For instance, if it has about 2.5" of drift at 200 yards, then by the rule of squares at 400 it'll have 2x2=4, 4x2.5"= 10". You can see how this agrees with the chart.

There's a longer explanation here:

http://www.6mmbr.com/winddrift.html

Sierra has a section on wind in the back of their manual, they have most of it online here. Unfortunately, the charts aren't online.

http://www.exteriorballistics.com/ebexplained/4th/532.cfm

 

[230RN]

Hatcher's Notebook, p 403, see attachment.

Note the (T - Tv) term. The wind deflection is greatest when the bullet is slowing down the most compared to its slowdown in a vacuum. Counterintuitive, but that's the way things are, and is why there is always debate about it.

T and Tv can be derived for a particular bullet and initial velocity from various tables (e.g., Ingall's tables) and some ballistic programs.

Terry, 230RN

For those having interest, there is also a listing of the errors in Hatcher's Notebook:
http://yarchive.net/gun/hatchers_notebook.html

 

[allaroundhunter]

For instance, a bullet drifts 1" at 100 yards and you want to know how far it would drift at 600 yards. Since 600 is 6 times 100, you square the distance difference. 6x6 =36 so the bullet will drift about 36 times the distance it did at 100 yards, or 36".

Not to split too many hairs, but the difference between 100 yards and 600 yards is 500 yards. The distance difference of 500 yards is 5 times the initial distance of 100 yards. Your wind drift multiplication factor should be 25 times (5^2=25)

 

[1-1 Banger]

When I went to sniper school, I was taught that the last 1/3 has the most effect on the round. To be safe, when I make my wind calls I always do the first and the last 1/3

 

[jmr40]

The bullet is fastest right at the muzzle. Wind does not have as much time to push it. At extreme long range the bullets speed has slowed considerably giving wind more time to work on it. It all happens pretty fast, but it could take 3X or 4X longer for a bullet to travel from 900 yards to 1000 yards than it takes to travel from the muzzle to 100 yards

 

[45_auto]

The distance difference of 500 yards is 5 times the initial distance of 100 yards. Your wind drift multiplication factor should be 25 times (5^2=25)

Nope, 600 is 6 times as far as 100. Multiplication factor is 6^2 = 36. I obviously didn't explain it very well, hard to do in a couple of sentences on a forum. Research is your friend!

 

[JPG19]

Thanks for all the info, folks! I certainly learned something today. You gotta love THR.

 

[allaroundhunter]

Nope, 600 is 6 times as far as 100. Multiplication factor is 6^2 = 36. I obviously didn't explain it very well, hard to do in a couple of sentences on a forum. Research is your friend!

Sorry, I read the difference in distance as subtracting the two values. I gotcha now, though. And forget this just being hard to explain in a few sentences on a forum, it's hard to explain in general!

 

[taliv]

it is actually rocket science

 

[USSR]

The debate over which wind flag to watch has been going on for a long, long time.

Don

 

[taliv]

yeah, it is kind of funny.

shooting XTC and F class, i learned to watch for the prevailing wind.

that is, look at the flags, shoot my sighters, and see which wind flag matched the bullet hole in the paper and call that the prevailing wind.

problem is, it's not always the same flag. sometimes it's a close one. sometimes a far one. sometimes it's on the left side, sometimes it's on the right side.

some guys seem to get blinders on and are just positive there's some rule of thumb they can use to always figure out the wind. it's amazing how these sort of guidelines come out of the woodwork and all assume that the wind is moving the same speed in the same direction as one monolithic mass between you and the target. my experience is as much as 1/4 of the time, the near wind and far wind are moving in opposite directions, due to the hills around here. how do you put that in a rule?

 

[allaroundhunter]

how do you put that in a rule?

Turn 90 degrees so that one wind is at your back and one is coming at you

 

[USSR]

...my experience is as much as 1/4 of the time, the near wind and far wind are moving in opposite directions, due to the hills around here. how do you put that in a rule?

Yep, same thing at Bodines, PA where I shot. Reading the wind is an art, not a science. Only thing I really learned that helped me was: shoot fast before the conditions change.

Don