Ballistic Factors that Most Affect Bullet Wind Drift

Wind is a fact of life, so when shooting long distances, wind drift is also a fact of life. Earlier, I posted on how to calculate and adjust for the wind  (Post 1 & Post 2). But is there anything we can do to reduce the wind affects on our bullets?

First, I will admit that I am not a long range shooter and rarely practice farther than 400 yards. I practice enough to know my limitations, but I prefer to get close because that is my favorite part of the hunt.

What do I mean by close? The last two animals I harvested were a mule deer buck at 20 yards (very close with muzzleloader) and a cow elk at 115 yards with a rifle. The ability for getting close depends upon many factors, but where I hunt in the mountains, long range shots are usually available, but the wind is often an important factor even as close as 200 yards.

So, to reduce the wind drift of my bullet, do I need to buy another rifle or do I just need to use a more streamlined bullet or a higher muzzle velocity? What can we do to reduce the wind drift on our bullets?

To start, we need to know which ballistic factors have the largest affect on a bullet’s wind drift. To identify those factors, I used the Point Blank ballistics calculator and entered the data for my 7 mm Remington Magnum.

Ballistic  &  Environmental Assumptions:

  • Bullet – 7mm Remington Magnum with 160 grain bullet (Federal Premium Vital Shok with Barnes Triple Shock X-Bullet)
  • Bullet B.C. = 0.443*
  • Muzzle velocity = 2,940 fps
  • Scope Height = 1.5 inches
  • Zero at 200 yards
  • Wind 10 mph @ 90°
  • Setting – Elevation 5,600 feet, 60°F

* I originally used BC of 0.508 for the test. But since Barnes Bullets told me that Federal calculated the BC at 0.508 at 100 yards and Barnes calculated the BC at 0.443 at 300 yards. So at distances of 300 yards or more, calculated bullet velocity, drop and drift would be more accurate with the lower BC.

By using a ballistics calculator and changing one of the factors at a time, we can see which factors have the largest affect on wind drift. Obviously, part of this process is purely an academic exercise, because we can’t really change bullet weight without affecting muzzle velocity and the ballistics coefficient, but it does help to demonstrate which factors affect wind drift the most. I was most interested in the affect of a better BC or faster muzzle velocity, but also included environmental factors such as altitude and temperature.

Factors Used to Calculate Bullet Wind Drift:

  • Ballistics Coefficient of Bullet (BC)
  • Bullet Velocity
  • Bullet weight
  • Elevation
  • Temperature
  • Distance

For the test, each factor was changed by 10% to see the effect on wind drift. Table 1, shows each factor, the original ballistics data, the 10% that was added or subtracted for the Test Data used to test each factor. Wind Drift is the calculated wind drift (inches) and the Percent Change is the difference from the original wind drift calculation of 15.64 inches at 500 yards. Table 1 is ranked by the Percent Change.

Table 1. Factors that Decrease Bullet Wind Drift

Factor Original Data 10% Test Data Wind Drift Percent Change
Distance  500 yards 50 yards 450 yards   12.44     -20.5%
Velocity  2,940 fps  294 fps 3,234 fps   13.52     -13.6%
B.C.    0.443  0.0443  0.4873   13.99     -10.5%
Temperature   288.7°K  28.9°K  317.6°K   13.99     -10.5%
Elevation  5,600 feet 560 feet 6,160 feet   15.26      -2.4%
Temperature     60°F    6°F   66°F   15.43      -1.3%
Temperature   15.56°C  1.6°C  17.1°C   15.57      -0.4%
Bullet Weight  160 grains 16 grains 176 grains   15.64        0%

The largest change in wind drift was produced from a 10% decrease in the distance. By decreasing the  distance from 500 yards to 450 yards, the wind drift decreased over 20%.

The next largest effect comes from a 10% increase in velocity which corresponds to a 13.6% decrease in wind drift. The Ballistics Coefficient of the bullet was the third ranked factor, with a 10% improvement of BC resulting in a 10.5% decrease in wind drift.

When considering temperature as a factor, which measure or scale of temperature should I use? Here in the U.S., we use the Fahrenheit scale, but most of the world uses the Celsius or Centigrade scale.

Science uses the Kelvin scale which is the measure from absolute zero. So,  60°F = 15.56°C = 288.71°K, so a 10% increase in each of these results in a very different test data of  66°F , 17.12°C (62.8°F) and 317.58°K (112°F). It is interesting that a 10% increase in temperature (Kelvin) has the same affect on wind drift as a 10% improvement of BC, but I will never be elk hunting in 112°F.

Change in elevation had a small effect (2.4% decrease in wind drift from a 10% increase in elevation), but changes in elevation are not linear. As we climb, the air gets thinner and thinner so a 10% change at 10,000 feet would have a larger effect (-4.3%).

We all know that a heavier bullet should resist wind drift more than a light bullet, but the ballistics calculator shows no difference when muzzle velocity and BC are kept constant. Generally, heavier bullets of the same design have better BCs, so calculated wind drift would be less.

Getting Close is the Best Way to Combat Wind

So the best thing we can do to combat wind drift is to get closer to the target. The next best thing is to use a load with a higher muzzle velocity and the third factor is to use a bullet with better ballistics coefficient.

I am very happy with the accuracy and terminal ballistics of my current ammo, but since Federal no longer produces the Premium Vital Shok with the Barnes Triple Shock X-Bullet, I will be looking for a new bullet after I use my last few boxes.

I will continue to get as close as possible before taking a shot and I will be looking for a different cartridge with a bullet in the 160 grain range with a faster muzzle velocity and/or a better ballistics coefficient. I like the all Copper Bullets. Any Suggestions?

 

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Comments

  1. Jeff Hooper says:

    Barnes bullets shoot better than anything I’ve put down the bore. I use the 290 grain bullet with 120 grains ff black powder in a Green Mountain 28″ side lock and it out shoots my Knight 24″ inline with less drop at 200 yards.

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