Let me start off by saying, all the wind formulas out there are more or less correct. As long as you are matching up the right bullet to the right formula or the correct constant itís easy to get close. After all itís just math and guys a lot smarter than I am have crunched the numbers over years to establish these formulas we use.
The question becomes, ďare you using the correct formula for your system and application?Ē
Unfortunately many donít realize these formulas are written in a way where key components are missing. Information like the bullet and muzzle velocity used to derive the answer. So what ends up happening is guys will stumble upon a formula meant for a Service Rifle shooter using a 5.56 and try to apply that to 7.62. Sure you might get lucky here and there, but you have no way of knowing what is luck and what is actually a solid call. Ballistic computers make life much easier and they guarantee the call is based off the actual round fired and not some hold over from 1978. But weíll discuss using a ballistic computer later.
The most common formula used is:
Range in 100 yards x Wind Speed in MPH / Wind Constant = MOA Hold
The MOA Based Constant(s) are:
100 to 500 yards = 15
600 yards = 14
700 to 800 yards = 13
900 yards = 12
1000 yards = 11
Mil Based Constant(s)
<= 500 = 45
600 = 43
700 = 41
800 = 39
900 = 38
1000 = 37
This formula you would use for a 175gr 308, in order to get the constant for other calibers you must work the formula backwards based off the actual hold you used. People will argue that these formulas are good for field use, however in 2012 itís much easier to derive drift using a computer. In order to determine the correct constant for your actual bullet, you would use the following formula.
Range in 100 yard x Wind Speed in MPH / Corrected Wind Hold = True Constant
As you can see, you need an actual hold in order to establish the constant. So the question becomes, how practical is this formula in the field ? Most tend to use this to build a drift chart after the Trial and Error method has run its course. That points to an labor intensive way of getting data. Lots of rounds downrange.
We have other formulas to work with, field expedient ďrule of thumbĒ methods of determining the appropriate hold on the fly. They all work so you can do the math quickly in your head. They still require memorization so they can be retrieved prior to the shot. In other words, you have to have it on hand, so you can quickly calculate the adjustment necessary for the shot.
The British Method
One of the older rule of thumb shortcuts is called the British Method. This based off a full value wind at 10MPH making it a bit easier to adjust the wind value when it is not exactly 10 MPH.
So then using 10 MPH as our base wind you use the following shortcut. Our base wind speed in this case equal 1 MOA per 100 yards.
Here is an example of the British Method in practice.
Range 600, velocity 10mph = 6 moa
Range 600, velocity 5 mph = 3 moa
Range 600, velocity 2-3mph=1.5 moa
Range 600, velocity 20 mph = 12 moa
The baselines for the wind are as follows:
2 - 3 MPH = Light Winds
5 MPH = Medium Winds
10 MPH = Base Wind
20 MPH = Heavy Winds
To convert any MOA based solutions to Mils you can divide your MOA answer by 3.43. This is the correct conversion factor when going from MOA to Mils or back.
The Rule of Nines
Similar to the British Method, you simply take the wind speed and multiply it by the range in 100 meter increments and then using the base value, 9 = .25 Mils, 18 = 5. Mils, and 36 = 1 Mil, you have the following example.
7 MPH @ 500m = 35, here 35 is close to 36 so you would use a light 1 Mil for the hold. Its not perfect but close enough for government work. If you had a 3 MPH wind at the same range the answer would be 3 x 5 = 15 which is close to 18 so a light .5 mils hold. Is this not 1/4 minute perfect, but it will solve a minute of man problem in the field.
BC Based Method
The BC method simply states that 308 has the BC of .4 so you use 4 MPH. If you use a bullet with a different BC that first number is the value you use for the wind speed. So a 5.56 would use 3 MPH based off the .3 BC for the bullet.
100 = .1 mil @ 4 mph
200 = .2 mil
300 = .3 mil
400 = .4 mil
500 = .5 mil
After 500 yards the velocity is changing you so have to adjust the values to account for this. It jumps up by .1 mils here.
The numbers continue this way.
600 = .7 mil
700 = .8 mil
800 = .9 mil
900 = 1 mil
100 - 1.1 mil
Pretty simple you just change the MPH value for .1 mils based off the bulletís BC. Itís not perfect, but itís fast and easier to remember than doing a long hand math formula in the field.
Now there are more formulas, also you can find a variety of drift charts for the common bullets used by both civilian and military shooters. Most pre-made Databooks have drift charts in them based off the first formula found here. They will give you the bullet and muzzle velocity used to determine the drift and are an excellent starting point. Most of this is just a point of reference and not a fact of the matter. The rifle system we are using, the muzzle velocity, bullet, etc all play a roll in what our actual hold will be. No to mention the shooter. The human factor can invalid a scientific number simply by how we press the trigger. If you are not following the Fundamentals of Marksmanship, you can easily change the value of your wind hold.
Theses are the most common formulas used and rules of thumbs out there. However using a ballistic program is some much easier and the information is based on your system. By going to JBM online and putting in your actual bullet, muzzle velocity and location you can tailor this information to your system and get very accurate wind holds for free.
The point with a lot of this is, if you are doing long hand math in the field you are dead wrong. It's too late to try and use this stuff above in the field. You can make charts off the information here, and you can print those charts to carry. But that requires prior proper planning. As noted with the constants. In order to have the correct constant for your system you need to have the correct hold ahead of time. So if you know the correct hold, why do you need the formula ?
I have some charts and some different things graphed out, but many of them are linked on the internet. A good example of this is the wind rose. The terminology used when calling the wind direction is very confusing. When people say it is a "half value wind" that does not mean it is 1/2 the amount of a full value hold, but really half the distance on the clock system which means a 75% hold and not a 50% one.
Look this modified Wind Rose that I created as the values are bit more telling then what people are used to calling.
Here is an example of a wind table for field shooting. Prior, Proper, Planning, Prevents piss poor Performance ! The more you do before you hit the range or go out to field, the better you will be.
Look at the 4MPH column... the BC method is there.
Do the work before, because if you're trying to work a formula when it's time to take the shot, you've already missed.