September 23, 2010
By Maj. John Plaster
In order to interpret and apply wind compensation correctly, you have to determine the angle of the wind; how it flows across the bullet will determine the amount of drift. A tail wind or head wind will have no value; they have essentially no effect on a bullet's flight. A direct crosswind, which blows from 90 degrees into the path of the bullet, is called a "full" wind because the full effect of the wind is experienced.
An oblique wind of 45 degrees, from right or left, has not a one-half value, but a three-quarters value. It has a 75 percent effect, even though the angle is only halfway between no effect and full effect. Most shooters initially have trouble getting this straight in their heads. The effect is not proportional because of the aerodynamics of a bullet in flight. Just remember that halfway between full and zero effect is three-quarters. Benchrest shooters use even finer values and split the wind for exact aiming. I've included this to give you a better feel for how quickly the wind has an effect once a bullet is other than at tail or head. Once it's just 15 degrees right or left, already a quarter of the wind value must be used when compensating.
Shooting into the Wind
To shoot accurately into a wind, compensate by holding or aiming in the direction the wind is coming from. As the bullet travels downrange, it drifts into your target. In order for this to work, however, you must know exactly how far to compensate.
The accompanying ballistic tables show wind drift for several police and military sniper loads, which you can compare to your favorite hunting loads. Although several wind speeds are listed, the most important is the 10 mph listing, I believe, because once memorized, it's easiest to compute in your head. Just about anything can be divided or multiplied when you start with a factor of 10.
Note that compensation doubles as wind speed doubles--the necessary compensation for a 20 mph wind is twice that of a 10 mph wind, and five mph is half that of 10 mph. But the differences in distances are not proportional: compensation for 600 yards is much more than twice that of 300 yards. This is because the farther the bullet goes, the more it slows down and the worse the effect becomes. In a way, this is similar to how a bullet starts to plunge at long range, when its path becomes a sharp arc.
But now, at last, we're ready to bring together ballistic data and wind values and compensation. It's really quite simple.
First, determine which direction the wind is blowing in respect to a line between you and your target. For the sake of illustration, let's say it's 90 degrees and, as already seen, that would make it a "full" wind. Next, determine the speed of that wind--we'll say it's five mph. Finally, you estimate your target is 600 yards away. You're using Federal .308 BTHP Match.
Looking at the table, you find that the required compensation is 16.1 inches. The compensation on all the tables reflects a full value. Since your scope has an adjustable windage knob, you dial in the equivalent of 16.1 inches at 600 yards; since 1 MOA equals six inches at that range, you rotate it 2.75 MOA into the wind. And because your scope has 1⁄4 MOA positive clicks, you turn it 11 clicks. Having made the adjustment, you aim dead-on, let off a good shot, and score a perfect hit.
If your scope lacked a windage knob, you would have looked at the target, determined that 16.1 inches is the width of a fit man at the hip, and held this far into the wind, aimed and engaged, again with perfect results.
But what about other than full-value crosswinds? Just factor in the value when determining the compensation. Let's try another example. You know the wind is 15 mph, coming on at a 45-degree oblique, and the range is 800 yards. Again you're using a Federal .308 Match ammo. The table says full compensation would be 96.1 inches, but we will only use three-quarters of that because the wind is oblique at 45 degrees. Three-quarters of 96 inches is 72 inches. So, if you have a windage knob you realize that 1 MOA equals eight inches at 800 yards; therefore, you divide 72 by eight, which equals nine, and you click off nine MOA on your scope, or 36 clicks. On another scope, you'd hold into the wind what you estimate to be 72 inches from your target--about the height of a man.
Where shooting into the wind gets tricky is when it's gusting or you must deal with several winds. Old-time shooters will tell you not to wait for pauses during a steady wind, that you'll have much better results shooting into a predictable wind than hoping a short calm lasts long enough for your bullet to reach the target.
Strong gusts require timing your shot. When faced by two winds, try to time your shot so it's fired during the slower or the least gusting or the farther wind so there's less effect and a more predictable outcome. (This is getting pretty complex, but the reason you prefer shooting through a farther wind is that there's less remaining flight time to be affected by the wind.)
When shooting in mountains, don't be concerned by updrafts. Where they exist, these winds are very shallow and your bullet will pass through too quickly to make much difference.
USMC Wind Adjustment Method
For those of you having a boundless desire for more information, I've included an old U.S. Marine Corps method for computing sight changes when firing in the wind. The USMC has been using this windage adjustment method since the days of the 1903-A3 Springfield.
After determining wind direction and speed, use the following formula:
Range in 100 Yds. x Speed in MPH/15 (math constant)= MOA Windage
For instance, your target is 300 yards away, and there's a 10 MPH wind:
3 x 10 = 30/15 = 2 MOA
Click-in the two minutes of angle in the direction of the wind and aim dead-on. This is a great formula--except it's only accurate at 500 yards or less. When your target is farther, the mathematical constant must increase, as shown below:
600 Yards: Divide by 14
700 Yards: Divide by 13
800 Yards: Divide by 13
900 Yards: Divide by 12
1,000 Yards: Divide by 11
Editor's Note: With slight modifications, this column was excerpted from the author's book, The Ultimate Sniper (Paladin Press, 1993; 303/443-7250.)