Blogger Curtis Stone from Captain of a Crew of One made some recent comments concerning our recently published article on Reading Wind and Mirage. He mentions the fact that wind will more greatly affect the trajectory of bullets as they continue down range.
One thing they weren’t very clear about: Wind effects on bullets over varying distances is not linear because the flight times are not linear.
Due to the drag of the atmosphere through which the bullet is traveling, the velocity of a bullet is constantly decreasing throughout its flight.
Therefore, on a 600 yard shot, the bullet is moving faster during the first 100 yards of travel than during the final 100 yards. Because it takes longer for the bullet to travel that last 100 yards than it does the first, the wind has a longer time to act upon the bullet’s path during the final 100 yards.
This is especially true under strong wind conditions.
What that means is you can’t just take your 100 yard correction for the prevailing winds and multiply by 6 to get an accurate 600 yard correction.
Curtis is absolutely right. We weren’t very clear on this topic in our earlier article, but there is a very good reason we multiply by wind speed and not distance in the examples we use. The effect of a full value wind increases as the bullet slows down. This is part of the reason why it is important to have a chart that calculates the drift at a given distance.
You can multiply by the wind speed, but you can’t multiply by the distance. For example, a 5 mph breeze might move a .223 bullet only a half inch at 100 yards. But you cannot multiply that by 6 to get 3 inches at 600 yards. That same 5 mph breeze will blow our little .223 round more than 45 inches at 600 yards. How much of a difference does that make? Curtis breaks it down in his example:
Using the match ammo that I generally prefer: 77gr Sierra Match King HPBT at 2750 fps at the muzzle, the correction for a “full value” 20mph wind at 100 yards is 1.75moa. If I just multiply by six, I get 10.5moa correction at 600 yards. In reality the correction at 600 yards should be 14.5moa. If I just tried to multiply the 100 yard correction by 6, my point of impact at 600 yards would be off by a full 4 minutes of angle or 24″. That would take a perfect center “X” hit out to the 6 ring on a standard NRA target. If you were shooting for a bad guy at that range, it would mean a clean miss…unless the bad guy was unusually hefty in which case you might wing him. [Emphasis ours]
The best way to prepare to compensate for wind at varying distances is to have the drift for your load already calculated for various distances (I like to use 50 yard increments) with a 1 mph breeze. In this manner, all you’ll need to do is select the appropriate distance, note the drift for a 1mph wind, and multiply by your measured wind speed and value.
This gets more complicated if you have wind breaks or shifting winds. Camp Swift, for example, is notoriously difficult to shoot due to the sometimes inconsistent and shifting winds there. You may have a 5 mph wind blowing one way near the shooting line, and an 8 mph wind blowing the opposite direction near the target. This seems like impossible conditions, but once you realize that it is the wind speed and direction in the final 200 yards leading up to your target that will have the greatest effect on your round, it’s not difficult to dial in your wind dope.
We’ll have more on this topic later when we address reading wind and mirage during rifle competitions.