January 04, 2011
Wind is the varmint shooter's greatest enemy, and the search for better "wind-bucking" bullets is never-ending.
If you've embarked on that search, be sure not to confuse a bullet's "wind resistance" with its "air resistance."
In flight, bullets are affected by three separate forces: gravity, air resistance and wind. Gravity pulls down on the bullet, causing it to drop below the line of sight. Air resistance (or "drag") slows the bullet with a force proportional to the square of the velocity. Wind makes the projectile drift from its normal still-air trajectory.
There are two key elements to understanding a bullet's susceptibility to wind drift. The first is its ballistic coefficient (BC), which combines the air resistance of the bullet shape (the drag coefficient) with its sectional density (the ratio of its frontal surface area to bullet mass), and is expressed by a figure providing the ratio of its ballistic efficiency compared to a standard theoretical reference projectile.
Sporting bullets have BCs in the range 0.12 to slightly over 1.00, with 1.00 being the most aerodynamic and 0.12 being the least. Bullets with higher ballistic coefficients have better air resistance, and the deceleration a bullet will experience due to drag is proportional to its BC.
In technical terms, wind drift is caused by drag. It is not, as is commonly thought, caused by wind pushing on the side of the bullet. Drag makes the bullet turn into the wind, keeping the center of air pressure on its nose.
This causes the bullet nose to angle into the wind and the base to be angled downwind. Since drag is the force slowing the bullet against the resistance of the air, the effect is to make it drift from its linear path in the downwind ("backward") direction. The greater the bullet's drag, the more it will drift.
Headwinds or tailwinds have similar but less obvious drag effects. A headwind will slightly increase the velocity of the bullet relative to the air it is passing though, which will increase the amount of drag and corresponding bullet drop.
A tailwind will reduce the drag and the bullet drop. In the real world, pure headwinds or tailwinds are rare since wind seldom is constant in force and direction and normally interacts with the terrain it is blowing over.
The second key element in a bullet's susceptibility to wind drift is its velocity. Here, unlike the complex physics of drag coefficients, the issue is simple. The longer the time any bullet is exposed to wind while in flight to the target, the more it will drift; the faster it gets there, the less it will drift.
While it is also true that drag increases as velocity increases, if you use bullets having high BCs to begin with, the benefit of the reduced flight time will outweigh any additional drag effect.
If you are seeking the maximum in wind resistance for varmint shooting, you need to pick the bullet with the highest ballistic coefficient (least drag) you can find for your cartridge of choice and push it to the highest velocity possible while maintaining the accuracy you need.