For the benefit of the rifle shooting community, I have created this table of common calibers’ wind constants. If you’d like to see another bullet on this table, just post the request in the comment section, and I will update the table (weight, G7 BC, and MV would be helpful). I have gotten it kicked off with 6 common calibers. I made this table using my highly accurate ballistics calculator, Sendit Ballistics. The table is right below. Further explanation is below the table.
|Caliber||Ranges Valid To (m)||Wind Constant (mil, meter, mph)||Wind Constant (moa, yard, mph)|
|5.56mm, M193, 55gr FMJ,||600||20||6.4|
|5.56mm, M262, 77 gr BTHP, 2740fps MV||800||27||8.6|
|5.56mm, M855, 62grn FMJ||600||23||7.3|
|6.5mm (CM), 140grn ELD-M||1000||45||14.3|
|7.62mm (300WM), Mk248Mod0, 190gr BTHP||1100||44||14|
|7.62mm, M118LR, 175gr BTHP, 2600fps MV||800||40||12.7|
|.22 LR, 40gr CCI Mini-mag, rifle||400||8||2.5|
Having a windage field constant in your back pocket is a great thing for serious shooters. A field constant means a single number which helps you come up with corrections based on a full value wind, and the given range.
The concept of a wind constant works on the following equation:
Meter Line * Windspeed / Constant = Correction
EXAMPLE: 5.56 62grn at 400 meters (meter line 4),
in a 6mph wind. Want MOA correction.
1. Constant = 7
2. Correction = 4 * 6 / 7 = 24/7 = 3.5MOA
- Meter Line is in hundreds, i.e. the Meter Line for 500m is “5”
- Windspeed is full value, interpolate for half values
- Constant is in such a unit that it gives you the desired correction
I made the table with two types of constants. The first row is for shooters who desire a mil correction, with meter distances, and mile per hour winds. The second column shows MOA corrections, for shooters shooting in yards, and using mile per hour winds.
You will notice that some of these constants are non-rounded numbers. I decided to err on the side of giving the rifleman the most information. If you don’t like “27,” you can make the call to round it to “25.”
The wind constant calculations are pretty much pure analytical science. I used the ballistics engine in Sendit Ballistics to produce the corrections for the given bullet and wind parameters. From there, it is simple algebra to calculate the wind constant (see the simple equation at the top of the page, we need to know 3 numbers to find the 4th). I modeled the wind constant at every hundred meters, and at 5, 10, and 20mph for each cartridge.
Past those calculations, there is some art involved. I first average all the wind constants to get a baseline number. This number is typically too low. The wind constants change drastically from 100m to 1000m. The constants are higher at shorter ranges, meaning that ultimately they produce smaller corrections for the riflemen in the field. Of course, the small wind constants at long range produce corrections which are too big at medium ranges. The key is to pick a reasonable range for the given cartridge, and then select a wind constant that will give adequate coverage for both medium and long ranges for that cartridge. It’s not too much of a problem if the shooter overcorrects by a large amount at shorter ranges; it amounts to centimeters.
Check out Sendit Ballistics for iOS if you are interested in an accurate, simple bullet trajectory calculator. I keep an updated page on the app here. The app is so accurate because it models the most important 3 degrees of freedom with painstaking detail. The app is designed for the field user, with a simple interface, range card mode, low light mode, and rapid corrections with minimal information. It is free to 400 meters, and only $4.99 to infinity.