A windage myth

A common misconception about windage is that the wind speed at the target is the most important. Superficially this seems reasonable since the slower a bullet travels the more it is deflected by the wind. An M855 is deflected by a 10MPH wind by 3.6 inches between 0 and 200 yards, by an additional 4.6 inches between 200 and 400 yards, and by an additional 6.4 inches between 400 and 600 yards which adds up to 14.6 inches but the actual 600 yard deflection is 39.3 inches as shown below. Something else is going on.

The point is that unlike a sailboat with an autopilot, a bullet doesn’t know where the target is. The wind blows it off course and so it is traveling sideways. After the first 200 yards it is traveling in the direction of the green tangent above. If there were no wind between 200 and 600 yards it would continue in a straight line with a total deflection of about 18 inches. The wind for the first 200 yards is responsible for 47% of the deflection. Similarly the wind between 200 and 400 yards is responsible for another 37% and the final 200 yards of wind at the target end is responsible for only 16% of the total windage deflection.

Pay more attention to the wind speed and direction at the shooting end for better results.

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