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Wind Wizardry: BC Wind Formula for Long Range Shooting

When shooting precision rifles, compensating for the wind pushing the bullet in flight is the most challenging part of hitting the target. Even the government and its ability to buy expensive toys don’t have a device that makes this process easy on shooters. Without getting into specifics on how to “read the wind”, there is an easy method to memorize and quickly recall a wind hold for your rifle. Being able to do so is beneficial if your battery gets drained or your phone overheats and you can’t pull up your ballistic app, or you forget your dope card and need to be able to make a call on the fly.

The level of precision needed and the amount of time to take a shot will determine how precise a shooter must be, but for field expediency, the BC Wind Formula is a handy thing to remember. There are other ways to determine correct wind holds for your rifle, but this is the cheapest and simplest way to do it if you don't have all the high-tech gear.

What do you do when your battery dies or your app stops working?

This formula has been used by a variety of shooters with different stories on its origins, but it's meant to work for most targets within a rifle’s effective range and be reasonably fast. If you’re trying to shoot a tiny target or engage one at extended range, you might need a more precise wind hold from a ballistic computer. However, for most shooters and scenarios this should get you on target or very close when combined with a good wind speed estimate. This is quick wind formula is also nice for a simple wind chart to remember for hunting.

The BC stands for ballistic coefficient, a calculated measurement of how efficient a bullet flies through the air. We will use a projectile's BC as the basis of this formula. There are G1 and G7 models with different numbers for the same bullet, however, we will use the G1 number. This number is advertised by most bullet manufacturers based on their own testing, as well as available in ballistic solvers and bullet libraries. That being said, the best way to know the applicable number is to develop a ballistic profile for your rifle and ammunition with verified shot data. The B.C. wind formula is based around using Mil-Radian (MRAD) scopes and holds but can be converted to Minute of Angle (MOA). The actual math would be to multiply the result by 3.438, but in a hurry multiplying by 3.5 works well.

To Calculate

  1. We start with the target ranges, converting the distance to decimals by moving the decimal place three places to the left. Meaning, 300 yards becomes .3, 600 yards becomes .6, etc.
  2. Next we will use the first digit of the G1 BC for a bullet as the applicable wind mile per hour. If a bullet has a .400 G1 BC, our applicable wind is now 4 mph (which, for this example, makes the gun what we call a “4-mph gun”). If the G1 was .600 the values would equate to 6 mph.
  3. A slight correction factor might need to be applied around 5-700 yards by adding a tenth of a Mil-Radian for less efficient bullets. For cartridges with higher muzzle velocities and/or very high BC numbers, removing a tenth of a Mil-Radian might be necessary at mid-range. A ballistic calculator can be used to cross-check your math.
  4. If your BC is not an even number, say .350, rounding up or down can be done checked against calculated numbers to determine if a 3 or 4 mile per hour rule of thumb would be better.

Wind Values

Now that we have a standard to remember, such as a gun is a 4-mph gun, we can apply this to making a wind call. If the wind is blowing at 4-mph, then it's an easy recollection. Again, for field expediency, if the wind is greater or less than the “standard” number, we can multiple or divide the wind amount in order to have a wind call. Keep in mind, that this 4-mph wind value example is for a full wind coming from 9 o'clock. If you see more or less wind, coming from a different angle, just multiply or divide the wind value accordingly.

For example, if we observe a 2-mph wind effect on a 500-yard target, we can remember that the 4-mph hold would be .5 Mils, and half of that would be .25 Mils. If the wind increases to 8-mph, our hold would instead become 1 Mil.

BC Wind Formula compared to real-world numbers.

In the chart above are a series of different cartridges from .223 to .338 Lapua. The field expedient MPH is given, then actual numbers were listed below for comparison. Inside of 1,000 yards, the numbers are all very close to being within the margin of error most shooters would have when making a wind call and breaking a shot. For example, .1 MRAD of deviation at 500 yards is only 1.8-inches, and 3.6-inches at 1,000 yards. For field-expedient math that is pretty close on the fly.

Without having to memorize every detail of a particular rifle, the BC wind formula provides shooters with the ability to quickly recall a wind hold. This is especially handy if a shooter has multiple rifles and/or ammunition loads. Finally, if you have time to set up a range card the BC wind formula can also be used to fill out multiple wind values for quick reference. Run the math and try this with your favorite rifle.

Wind chart created with B.C. Wind Formula.

 


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