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Bucking The Wind

By Mike Precure

We all know that the wind affects bullets in flight.  How much, and does it affect every bullet the same?  If not, why not, and how can we deal with it to improve our accuracy?

Accuracy and Precision:

First, let’s consider what accuracy is.  Precision is the mechanical consistency from shot to shot.  It’s the stiffness of the action, control of barrel harmonics, cartridge pressure consistency etc. all added together.  Smaller group sizes in absolutely predictable conditions is an indicator of precision.   Accuracy is the ability to put that precision where you want it to be – on target.  The bullet dispersion (group size above) centered on the intended target is accuracy.   No amount of rifle or reloading precision will overcome unpredictable accuracy and vice versa.  Because it changes and the shooter will have to overcome it, wind is a factor in accuracy but not precision.

We all intuitively understand the effect of gravity on a projectile in flight.  We learned this as children by throwing rocks, baseballs etc.  We instinctively compensate for the pull of gravity without thinking about it.  But very few of us can throw a rock far enough to need to account for wind.  Bullets in flight, unlike rocks we throw, are very much affected by wind.  They’re affected but we can’t actually see what’s happening to them and why.  It’s not something we intuitively understand in our nearly zero velocity world.  But, for the high velocity world of bullets in flight, it’s a very big deal.

Why bullets in flight drift:

As soon as there’s a slight breeze the impact point of a bullet will shift.  It will shift more as the wind approaches right angles to the bullets flight, as the wind speed rises and the longer the flight time the greater the effect.  How much will you have to compensate?

According to Bryan Litz in his marvelous (and thick!) handbook on external ballistics, Applied Ballistics for Long Range Shooting, wind drift is not really caused by the pressure of the wind on the side of the bullet.  The side pressure effect is an easy thing to imagine since we’re pretty slow moving and that’s how wind affects us.  Bullets, however, are affected by things we don’t ever experience.

What actually makes the bullet move is drag.  In a cross wind, the bullet nose will yaw into the wind just like a weather vane.  Drag is always pulling at the base of the bullet in line with its axis.  In a cross wind, when the bullet yaws, the drag is pulling at an angle that’s different from the bore line of the barrel, the direction we actually fired the bullet.  From our frame of reference, the bullet has drifted.

How can we measure and predict drift to improve accuracy:

Measuring drag

Drag is a function of shape and speed.  Yes, drag changes depending continually on how hast the project is going at the time.  The reason is again outside the scope of this article, but it’s something to be aware of.  There’s a handy indicator of these factors available to us.  It’s called the ballistic coefficient or BC.  There are two common ways of expressing BC.

In the BC calculation, drag is expressed as a unit less number based on a comparison to a standard projectile.  In the G1 version (the one you’ll see most often), the standard projectile has a round nose and a flat base.  In the G7 version, the projectile looks more like a modern, low drag projectile with a front point and a tapering tail (often called a boat tail).  The G7 will give you a better indicator of performance over a velocity range for a high performance bullet but the G1 is by far the most common expression you’ll see.

        
 
      



Since the ballistic coefficient tells you about drag, it also tells you about wind drift.

Measuring and predicting drift

If a bullet were flying in a vacuum it would have no drag influence on the way to the target.  Introduce air and drag will slow the bullet over time depending on how much air you introduce.  This is why air pressure and altitude matter with wind drift.  Denser air means more drag.

The best indicator of drag is the actual time of flight compared to the time of flight in a vacuum.  This is called the “lag time”.  The difference in those times and knowing the wind direction and speed will tell you what your drift will be.  Remember, air density and drag (BC) will already be factored in when you know the actual time of flight so it’s included in the lag time no longer part of the drift equation I’m expressing below.

You can easily calculate wind drift with this simple equation:

Wind Drift (Inches) = (Actual Flight Time - Time of Flight in Vacuum) * 17.6 * Wind Speed MPH

Given:

  • 17.6 is to accommodate wind speed in MPH.
  • Actual Flight Time - Time of Flight in Vacuum = Lag Time
  • Wind Speed in MPH assumes the 90 agree component of the wind.  Some conversion factor is used for a non 90 degree to flight path wind.  (You’ll need to apply a vector calculation for a wind measured outside of 90 degrees).

Example:

 

 

Bullet

 

Ballistic

Coefficient

Muzzle

Velocity

(FPS)

 

Distance

(YDS)

 

Flight

Time

Flight

Time in

Vacuum

Wind

MPH at

90⁰

 

Drift

inches

.204 32 Gr Vmax

.21 (G1)

4000

700

.98

.53

5

40.04

.204 40 GR Vmax

.27 (G1)

3900

700

.78

.54

5

20.90

 

See the difference?  The difference in flight times between the 32 and the 40 gr projectiles makes a huge difference in a cross wind.  Though the BC isn’t actually used in the calculation (it’s included in the flight time), it’s such a handy way to predict behavior I included it.

You’ll often hear people say they are selecting a heavier bullet that ‘bucks the wind’ better.  The statement implies that weight resists wind better.  Is this true?  No, not exactly.  A heavier bullet for a certain bore diameter is either made of a more dense material or it’s longer.  It all depends on a ratio of weight and shape (drag).  That ration is expressed in the BC.  Let’s consider two bullets of different weights in the same caliber.  I kept the velocity the same to keep things simple:

 

 

Bullet

 

Ballistic

Coefficient

Muzzle

Velocity

(FPS)

 

Distance

(YDS)

 

Flight

Time

Flight

Time in

Vacuum

Wind

MPH at

90⁰

 

Drift

inches

.264 140 Gr VLD

.61 (G1)

2900

700

.86

.72

5

11.8

.264 160 Gr RN

.28 (G1)

2900

700

1.08

.72

5

31.3

 

Did selecting a heavier bullet “buck the wind” better?  No.  It drifted a whopping twenty inches farther even though I unrealistically kept their muzzle velocities the same!  Now that you understand how this really works you can see how and why the BC is actually a better indicator of wind drift performance than weight alone since the BC takes both weight and drag into account.

As you can now plainly see, the whole phrase “bucking the wind” really makes no sense at all.

What can be done to improve accuracy in a cross wind:

In the examples above I’ve treated wind as a constant and predictable force.  It’s not.  It gusts, shifts directions, swirls around hills and valleys, it’s a different speed at different heights above the ground and it comes from different directions along a bullets flight path.  If you spend a lot of time learning to read the signs and can accurately predict all the permutations your scores will go up dramatically.  But can you do something to help yourself, regardless of your ability.  Yes, you can.

You cannot eliminate or reduce the wind.  All you can do is mitigate its effect.  Using the above examples you can see that by reducing the lag time you will reduce the drift.  By reducing the drift you mitigate the uncertainty of your ability to precisely know the exact wind calculation for each shot.

Let’s use the 700 yard .264 example above:

 

 

Bullet

 

Ballistic

Coefficient

Wind

MPH at

90⁰

 

Drift

inches

 

Wind

Uncertainty

 

Drift

Uncertainty

 

 

Spread

.264 140 Gr VLD

.61 (G1)

5

11.8

20%

14.6 to 9.44

5.2

.264 160 Gr RN

.28 (G1)

5

31.3

20%

37.6 to 25.04

12.2

 

So what’s happening?  The bullet with the higher BC (less drag and therefore less lag time) is actually far more predictable in a variable condition.  By selecting the 140 Gr VLD over the 160 Gr RN your shot to shot variability is cut more than half with no change at all in your equipment or skill.  It’s just plain easier.

Conclusion:

The term “bucks the wind” in its common use is really not accurate at all.  Bullets do not buck or resist the wind.  Different bullets do react differently to it, and the best and most complete expression of how well or how poorly they react is the ballistics coefficient, not the weight.

If you want to improve your shooting in real world, outdoor conditions, choose a cartridge and bullet combination based on velocity and ballistic coefficient.  Your scores can’t help but improve, even before your wind reading skills mature.