Timely

On Wednesday, I wrote about the Atlantic Fleet Rifle match, how well I did reading the wind and how squirrely the winds were on EIC day from the long line.

On the same day, Cheaper Than Dirt blogged about the finer points of reading and compensating for wind and mirage.

I just stumbled across their post today, but the timing was pretty interesting.  What a coinkydink.

One thing that I would add to their discussion:

They mentioned that the Ballistic Coefficient of a bullet has an impact on the amount of effect the wind will have on it for a given flight time.

All bullets have a ballistic coefficient that is usually computed by the manufacturer. This number, combined with the flight time of the bullet, can help you determine how much your bullet will be affected by a given wind.

They also talked about using the 1mph standard to derive the amount of correction needed for a specific wind velocity and direction at a given range.

Our chart shows that M2 match ammunition for an M1 Garand from American Eagle will drift approximately 5.8 inches at 600 yards with a full value wind at 1 mph. If we actually have a 10 mph wind blowing in at a 45 degree angle (1:30 o’clock) we assign it a value of 3/4 and do the math (5.8 inches X 10 mph X .75) to arrive at 43.5 inches of drift.

But 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.

For 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.

Even the difference between 300 and 600 yards is significant.  The correction for a full value 15mph wind at 300 yards with the same ammo would be 4.5moa.  Multiplying by two for 600 yards would give a correction of 9moa, when, in reality the correct adjustment should be 10.75moa.  The difference is less significant with the lower winds and less difference in range, but it still would change the point of impact by 10.5″, or would throw my perfect center X shot out to the 8 ring.

So, what I would add to the CTD blog’s discussion is that it is important to know (or have written down) the 1mph standard for your ammo for various distances.  Don’t just know one and expect to be able to calculate the rest with basic math.

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