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# Long range wind

#8
03-02-2006, 11:55 AM
 Platinum Member Join Date: Dec 2005 Posts: 2,483
Re: Long range wind

Idaho Roy, first the easy part. The Dwyer Wind Meter I mentioned can be seen at

http://www.stumpfballoons.com/wind.htm

and

http://store.yahoo.com/cspoutdoors/dwyerwindmeter.html

selling for \$22 and \$15. Just hold their back side into the wind and read the speed next to the floating foam ball. Hold your fore finger over the top for high winds. Such a deal.

"Magic numbers" are the number you use at each range for each load (bullet at some muzzle velocity). Each one has to be calculated then memorized or written on a label taped to the back of the wind meter (that's how I used them). Multiply range in hundreds by wind speed in mph, then divide that answer by the range's magic number.

Example: You've calculated magic numbers for a given bullet as follows:

200 yards = 18
300 yards = 17
and when you get out to the longest range you'll use...
1100 yards = 11
1200 yards = 10

"Magic numbers" can be also called a "wind constant" for a given bullet at a given muzzle velocity at a given range. As range increases, the wind constant gets smaller as wind drift increases in moa.

Let's say the wind's 14 mph from 2 o'clock and your \$5 laser rangefinder says the coyote's 850 yards away. Multiplying 8.5 times the 14 mph wind speed is about 120; dividing 120 by your load's 900 yard magic number of 13 you get 9.2. A 2 o'clock wind is worth 90 percent and 90% of 9.2 is about 8.3. So move your sight 8.3 moa into the wind and nail that prairie puppy pouncer pronto!!!

The wind's value from some angle is based on the sine of the angle. At 1 o'clock, the sine of 30 degrees is .5, at 1:30 or a 45 degree wind, the sine is .7, at 2 o'clock, the sine of 60 degrees is about .9 and at 3 o'clock the sine of 90 degrees is 1; a 3 o'clock wind is a full value wind. Same numbers apply from other clock numbers that have the same relative angle.

Hope this helps.
#9
03-02-2006, 07:17 PM
 Platinum Member Join Date: Dec 2005 Posts: 2,483
Re: Long range wind

Roy, here’s another way to get those “magic numbers” or range constants.

Let’s take a 30 caliber 180-gr. spitzer boattail leaving at 2960 fps at an altitude of 6000 feet. At each range below, the wind drift in inches is for a 10 mph cross wind from 3 o’clock.

At 400 yards, 8.3 inches, 2.1 MOA correction needed

At 800 yards, 36.9 inches, 4.6 MOA correction needed

At 1200 yards, 94.1 inches, 7.8 MOA correction needed

What’s needed is some number to divide the base formula’s answer (Range in hundreds by Wind in mph) by to get the correction for a full value wind.

At 400 yards, 4 x 10 = 40, so 40 / 2.1 MOA = 19, the magic number (or constant) for this load at 400 yards.

At 800 yards, 8 x 10 = 80, so 80 / 4.6 MOA = 17, the magic number (or constant) for this load at 800 yards.

At 1200 yards, 12 x 10 = 120, so 120 / 7.8 MOA = 15, the magic number (or constant) for this load at 1200 yards.

Run the wind calculation formula for each range and see what MOA value you come up with for each one. Again, the formula is Range (in hundreds of yards) times Wind (in mph) divided by the Magic number. Or: RxW/M
#10
03-02-2006, 08:07 PM
 Platinum Member Join Date: Jan 2004 Location: Blackfoot, Idaho Posts: 8,826
Re: Long range wind

Hmmm, I'll give it a go.

Was blown off the test range this morning. 90 degree wind was blowing the .243 100gr bullet 5 to 8 inches at 200 yds.

Only slightly less wind at about 10 degree off of line of sight had minimal effect this afternoon. Was quite supprised. This is the first time I have shot anything smaller than 338 in over a year.......
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#11
03-02-2006, 09:52 PM
 Platinum Member Join Date: Dec 2005 Posts: 2,483
Re: Long range wind

[ QUOTE ]
Was blown off the test range this morning. 90 degree wind was blowing the .243 100gr bullet 5 to 8 inches at 200 yds.

[/ QUOTE ]
That's not much wind. Here's a "windy" bit of reality.

When the US Palma Team started to shoot in the 1988 World Championships Team Match held in Sydney, Australia, the wind was coming from about 8-o'clock. Coming very, very hard. I laid down on the first relay of four on our 16-person team and my coach was talking with the other three behind their shooters. They were deciding how much wind to use on the 800-yard line. It was a toss-up between 27 or 29 minutes left correction for the arsenal 147-gr. bullets we were using in our .308 Win. rifles. 'Twas decided to go with 28. That's 112 clicks on my quarter-minute rear sight. He told me to load and be ready, so I did. Then put on 112 clicks left. "Shoot" he says so I did and called the shot a bit left and high as the wind was buffeting me around somewhat. Target went down then came up with the spotter a bit left and high of center. Such is life when you've got a good wind doper for a coach. My job was to shoot small groups. His job was to keep it in the middle.
#12
03-06-2006, 05:44 PM
 Bronze Member Join Date: Nov 2001 Location: Wyoming Posts: 92
Re: Long range wind

Well, I went out today for a little plinking with the .243AI... The breeze was a pleasant 20+ mph out of the west with gusts to 33 according to the Skywatch Meteos. Fortunately the balmy 57 degree temperature made it almost tolerable. About the only thing I really learned today is that at 20+ mph, I blow around way more than the bullet does! Anyone know where I can get a deal on a "portable" concrete bench and wind block?!
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#13
03-08-2006, 08:19 AM
 Gold Member Join Date: Oct 2004 Posts: 723
Re: Long range wind

Guys, it would be interesting to know what signs from vegetation, etc.. you look at for calculating the speed of wind ..... you can´t always have a wind meter with you.
#14
03-08-2006, 08:31 AM
 Platinum Member Join Date: Dec 2005 Posts: 2,483
Re: Long range wind

The best long range competitive rifle shooters use their spotting scope focused to see the mirage (heat waves) wrinkling across the field of view. By focusing the scope about 2/3rds the target distance, the wrinkles moving across are an exact image of what the wind is doing.

There are zillions of videos and printed words on the market that try to teach folks how to "read the wind." None of them work well.

The best way to learn how to do this is to have a very good shot ready to shoot at a 600 to 1000 yard target on a rifle range, then you look through your spotting scope, read the mirage then estimate how much windage correction is needed. You get excellent feedback when the target's spotted on both on how well you estimated the correction as well as how well the shooter performed. As the cross wind speed changes you'll get to change the amount of correction.

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