Advanced Marksmanship Wind Margin of Error

[Bro Mo]

I'm trying to learn more about ballistics and contemplating the margin of error allowable in calling wind and how that margin changes over distance.

As an example, the margin of error to hit an MOA target drops below 10% on a 175g 308win at ~500y, 140g 6.5mm Creedmoor at ~700 yards, and 112g 6mm Creedmoor at ~800 yards. The closer in, the margin of error gets greater and the farther out, the smaller margin of error allowable. A 50% increase in range with the same margin of error is pretty significant.

At 1000y, the margin for those three drop to 5%, 6%, and 8%, respectively. At 1000y, all three of those margins are pretty small and, to me, indicate that if I can't call the wind close enough with one, I likely can't with any.

When I see the math worked out, I feel that I better understand the anecdotal evidence of experience and how it relates to the importance of cartridge selection for combating wind.

All that said, was 10% a good estimate; what is a typical margin of error for reading wind?

 

[Dthomas3523]

I’d say the very best will get the wind call within 2mph. Most everyone else will be within about 3-5mph.

So, let’s say a 15mph wind. Average person will be 20-30% error. Which is a a big reason why you see 100% ipsc past 600yds at a lot of matches.

 

[Steel head]

I feel I’m in the 3-5 MPH camp most days especially in new conditions/locations.
At my normal spots where I have experience and dope I can call wind pretty well most days.

 

[JustSendit]

I’ll math the 500yd 175SMK target (mv 2650, custom drag curve)

That’s a .28mil width target, and for DA ~0,
For wind deflection:
1mph is .12
2mph is .24
3mph is .36

(0 Latitude, 0 DOF, no spin drift)

I would need to be within a 2mph with a wind call or I will miss a 1moa target at 500yds.

I’m not sure where your 10% was derived but I think if you can call within within 10% then your hit probability would be pretty damn good. But there’s other factors of course this is in a vacuum.

You may want to check out Applied Ballistics Analytics, there’s a feature called WEZ, Brain Litz discusses it in detail. I believe you are able find google and find resources.

Seemingly he’s quickly demonstrated what factors = hit percentage in a easy to understand method. What he does is kind of make an error budget between all the factors of shooting and the program spits out a hit probability overlay of reasonable expectations.

 

[Jack Master]

^^^^ What Justsendit said.
Accuracy and Precision for Long range shooting <-- Link
Brian Litz of Applied ballistics has developed a 6 degrees of freedom ballistic soft where that also allows the input of other variable. Shooter/rifle ability, Target size, Confidence in target distance and confidence in wind calling are a few. Here is a photo of the chart provided for the 6.5creedmoor.

Note the targets sizes. (larger for shooting inside 500yds) but at
Example;
-800 yards
-10" plate,
-knowing you are within +- 5 yards of the target distance
you could have 94% hits if you can call the wind to within 1mph but only 34% hits if you can call the wind within 5mph.
It was a great book to read.

 

[Sheldon N]

As an example, the margin of error to hit an MOA target drops below 10% on a 175g 308win at ~500y, 140g 6.5mm Creedmoor at ~700 yards, and 112g 6mm Creedmoor at ~800 yards. The closer in, the margin of error gets greater and the farther out, the smaller margin of error allowable. A 50% increase in range with the same margin of error is pretty significant.

I think you may have gotten the math or your underlying approach wrong based on the numbers above. A good way to look at how cartridge selection effects reasonable range for a probable hit is to run the 3 mph wind value on it.

Assumption is that you can call the wind direction and value within 3mph. Running a 3mph chart tells you how much you will miss by if you get the wind wrong by 3mph, whether that's the difference between 5 and 8mph wind or a 19 and 22mph wind.

You then pick the yardage on a drop chart where the wind drift reaches one half the target width (holding center of target, missing off edge). That's the yardage at which you miss if you get the wind wrong by 3mph.

So using your three cartridges above (308, 6.5 creed, 6 creed high velocity high BC) here are the rough yardages they miss at with a 3mph error.

2 MOA Target
450 yards (308)
525 yards (6.5)
650 yards (6mm high vel high BC)

If you want to hit a 1 MOA target, take the values above and cut them in half. That's correct... if you had to hit cold bore 1 MOA target with a fast 6 creed and 112gr bullet, odds are that right around 325 yards is the farthest where you could do it with some degree of confidence.

 

[Bro Mo]

So using your three cartridges above (308, 6.5 creed, 6 creed high velocity high BC) here are the rough yardages they miss at with a 3mph error.

2 MOA Target
450 yards (308)
525 yards (6.5)
650 yards (6mm high vel high BC)

If you want to hit a 1 MOA target, take the values above and cut them in half.

I also got ~225y for 1MOA for the 308 at 30% error for 10mph wind which is about the same. I'm sure there is more that is missing in the way I calculated it, but it doesn't seem super far off either.

 

[Sheldon N]

I also got ~225y for 1MOA for the 308 at 30% error for 10mph wind which is about the same. I'm sure there is more that is missing in the way I calculated it, but it doesn't seem super far off either.

Maybe the shift I would suggest is to avoid thinking of it in terms of % error, and instead think in mph error. The amount of miss is the same whether you called a 3mph wind that was actually a 6mph wind (100% error) or you called a 22 mph wind that was actually a 25mph wind (14% error).

 

[Bro Mo]

You may want to check out Applied Ballistics Analytics, there’s a feature called WEZ, Brain Litz discusses it in detail. I believe you are able find google and find resources.

A blog series on Precision Rifle Blog is what originally got me curious.
How Much Does it Matter

 

[Bro Mo]

Maybe the shift I would suggest is to avoid thinking of it in terms of % error, and instead think in mph error. The amount of miss is the same whether you called a 3mph wind that was actually a 6mph wind (100% error) or you called a 22 mph wind that was actually a 25mph wind (14% error).

I have so much to learn.

 

[C Renz]

Well good thing I'm carrying my kestrel everywhere cuz my wind skills need daily training! This stuff is good reading.