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Wind Drift

The biggest problem with wind drift under hunting conditions, is not the technicalities of how to calculate it, or what formula is best for doing so, but simply to reduce its effect on the shot you are going to take in the next couple of seconds as much as is within your power. 

The complexities of estimating wind drift under field conditions is something best left for discussions around the campfire when one is into the mellow time zone. Make no mistake, wind drift can bedevil months of careful planning and preparation and is often the cause of meat that has to be tracked instead of prepared.

Minimising wind drift should be done well in advance and is best dealt with at the stage when loads are developed and as early as the stage when bullets are purchased for assembling the loads. Any kind of shooting beyond 100 metres will require that one consider measures to minimise wind drift.

A good strategy would be to choose a load and bullet combination that will perform best and test it on the range under windy conditions at various distances. Then set a limit on the distance at which that combination can be used reliably, just as one would do with the limitations of trajectory of the caliber/load one uses. This will go a long way towards reducing distractions when one should be concentrating on shot placement, a clear line of sight and the many other considerations when the shot is about to break. Worrying about wind drift at this time is futile. In any case, it is virtually impossible to accurately estimate the average wind speed over the distance involved, the exact distance, the angle at which the bullet path and wind direction intersect, the humidity, and the barometric pressure.

There are many misconceptions around wind drift and how to minimise the effects thereof and the most common ones are:

In order to address these old wives tales and put the whole thing into perspective, one must first understand that the factors that cause wind drift are not really that complicated. (Some wag will no doubt pipe up that wind is the obvious one.) To ensure that we are clear on the issue, a couple of facts need restating. Apart from factors that we cannot control, such as actual wind speed, direction and atmospheric conditions, wind drift is governed by two factors that we can control: Bullet speed and bullet ballistic coefficient (BC).

Unfortunately these two factors are not really compatible. The better the BC becomes, the longer the bullet. The longer the bullet becomes, the heavier it is. Heavy bullets cannot be driven as fast as lighter ones. Catch twenty two. It boils down to finding the best possible combination of speed and BC. This gets tricky because all bullets lose proportionally more speed in the first part of the trajectory to the target than the last part. See the table below for a comparison.

1. Velocity Loss Over 300m and Wind Drift of Bullets of Differing BC Values

.308 Cal

MV

100 m

loss

200m

Loss

300m

Loss

Drift

BC

130 gr

3100

2740

360

2407

333

2099

308

10.88"

.323

150 gr

2900

2614

286

2346

268

2093

253

10.62"

.358

180 gr

2700

2472

228

2255

217

2049

206

9.52"

.431


It is important to note that changing the weight of a bullet, while maintaining the speed and BC figures, brings no change to the value of the calculations in the table above. It is clear that weight, sectional density, momentum and kinetic energy as single factors, play no role in determining how much drift a bullet will produce. Insofar as they are part of the BC and velocity equations, some do play a role. 

From the graphs below, we can see that increasing the BC in 10% increments, will reduce the drift by a closely corresponding percentage. Increasing the speed in 10% increments, results in a bigger gain in the reduction of drift. Increasing BC is of course easier to do than increasing speed as one simply selects a bullet with a better BC value than another.


To return to our list of old wives tales, the first four are proven false as none of the factors they mention play a role in determining drift. The fifth misconception gets a bit more complicated as there is a basis of truth buried in it. Wind drift has a vertical component as well as a horizontal component.

The horizontal component is roughly proportional to the wind speed and the square of the range and is unaffected by spin rate or gyroscopic stability(GS). The vertical component is roughly proportional to the wind speed and the range and is indeed affected by the GS of the bullet. GS increases if the twist rate of one barrel is tighter than another, or if a shorter bullet is used. In a right hand twist barrel, wind from the right will cause bullets to strike higher than those launched from a left hand twist barrel and vice versa. Therefore, if wind speed increases with wind from the right and a right hand twist barrel, drift to the left increases but bullet drop decreases.

In practise, the entire vertical dispersion / barrel twist connection can actually be ignored under hunting conditions as it is so small, it is not worth fretting over. The difference in vertical dispersion between no wind and a 35 km/h crosswind from any particular barrel is less than 13mm at 100 metres. The difference in vertical dispersion between a one in nine twist barrel and a one in twelve twist barrel in a 35 km/h wind is less than 4 mm at 100 metres. Depending on the wind direction and the direction of twist, the dispersion difference could be up or down.

In a nutshell then, we see that wind drift can be minimised by increasing speed or increasing BC. First prize is if one can increase both. With these facts at hand, let us look at some practical examples. On a recent springbuck hunt, a variety of calibres and bullets were used. On one of the days of the hunt, we had strong wind to contend with. The figures are given in table 4 for a 40km/h wind with three of the calibers used on the day. None of these bullets are shining examples of high BC numbers and especially the .224 would normally be assumed to be a real dog in the wind. It had a lot of speed going for it though, and performed well. The bullets used were: 40gr HV bullet in the 22x64, 139gr flat nosed jacketed lead bullet in the 7-08 and a round nosed flat base jacketed lead bullet in the 30-06.

The actual mechanism of wind drift is interesting as it is often assumed that it is the result of wind blowing on the side of the bullet. In reality the bullet vectors (turns) into the wind within a short distance after exiting the muzzle. The stronger the wind, the more acute the angle the bullet assumes into the wind. The result is that the drag on the base of the bullet is then offset more to the downwind side of the bullet path and drags the bullet in that direction. More on this subject can be seen here.

Based on the premise that an increase in speed as well as BC can only be beneficial in reducing wind drift, a practical comparison of the advantage of HV bullets is given in table 5. The caliber is 30-06 and the comparison is made between a 150 gr HV bullet, a jacketed lead 150 gr bullet and a 180 gr jacketed bullet at typical speeds for these bullets. Wind speed is taken at 25 km/h.

If one holds the point of view that shot placement is by far the most critical aspect of bringing home the venison, maximum reduction of wind drift is a valuable advantage to have in the bullet you load. Pick your load components with a view to increasing the ease of your shot placement, thereby reducing the distraction of wind and enjoy the hunt.

To your success,

Gerard Schultz

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GS Custom Bullets, situated in Port Elizabeth on the East Coast of South Africa, manufactures solid copper, turned, monolithic bullets for hunting and sport shooting. These bullets are used by hunters on several continents, hunting from the smallest of antelope to the largest of dangerous game, using the smooth HP bullet, as well as the more popular HV, FN and SP bullets with the patented drive band concept. GSC bullets are configured for the highest possible ballistic coefficients. SP bullets are mainly used for sport shooting. All GS Custom Bullets are moly coated.