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Old November 23, 2012, 05:34 PM   #1
1stmar
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Is Wind drift linear?

If a certain bc, velocity and wind generates 1" of drift at a hundred yards would half the wind value cause half the drift? Say a bc of .235, 3600 fps MV and 10mph= 1" of drift at 100, would a 5mph wind cause .5" of drift all other variables he same?
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Old November 23, 2012, 06:26 PM   #2
Bart B.
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Yes. Only half the high value wind's force is applied to the bullet for half of its speed.
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Old November 23, 2012, 06:29 PM   #3
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Thanks Bart
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Old November 23, 2012, 06:32 PM   #4
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Pretty close.

Base on you info, a 5 mph wind would drift the bullet .23 inchs at 100 yards, .47 @ 10 mph, .70 @ 15 mph, and .94 @ 20 mph.

That's an estimate, I don't know your altitude, hum. etc. but all else being equal its linear in the way you're looking at it.

There is a lot more to it, but there you have it.
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Old November 23, 2012, 09:17 PM   #5
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Pretty much linear at a given distance. As the distance increases, the velocity is less, so the farther out you go the more the drift for a given wind speed.
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Old November 24, 2012, 01:03 AM   #6
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A linear approximation is close enough.

In a mathematically perfect simulation, it is not linear, but until you get to extreme distance with extreme winds, linear is a perfectly acceptable way to model point of impact shifts. And by the time the effect would be noticeable, you would be shooting "beyond small arms capabilities" anyways.

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Old November 24, 2012, 05:06 AM   #7
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Kraigwy, I'll have check but I believe the hornady wind drift table showed a 1" drift in a 10mph wind with that bc and MV? They don't list other wind speeds, hence the question and I think the closest bc was .240. The edition is pretty old, are you sure about those values?
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Old November 24, 2012, 06:06 AM   #8
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Were in the hell do you guys shoot were the wind blows at the same speed and direction all the way to the target?
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Old November 24, 2012, 07:46 AM   #9
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There are several very good online ballistics calculators where you input bullet BC, bullet weight, muzzle velocity, altitude, relative humidity, air temperature, wind speed and angle, etc.

I tried Hornady's online calculator and plugged in the numbers for .22 long rifle standard velocity using a BC of 0.13.
5 mph crosswind = 8.1 inches of drift @ 200 yards
10 mph crosswind = 16.2 inches of drift @ 200 yards.

So yes, wind drift is pretty linear.

How did I know that the BC of a .22LR is 0.13? It's not published anywhere.

I plugged in different BC's into the calculator until I found one that made the calculator match published 100 yard velocities. It's not a perfect science. I couldn't find a single BC that made the calculator match both the 50 and 100 yard published velocities but they were close.
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Old November 24, 2012, 08:15 AM   #10
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This: "Were in the hell do you guys shoot were the wind blows at the same speed and direction all the way to the target?"


It is not unheard of to get no wind, and right, and left, all at the same time. Happens more at longer ranges, of course (I am thinking 600 yards). Or just one short gust that only affects the bullet for a short distance.

A good spotting scope, focused on the mirage, can help read those winds. Wind flags, watching the grasses, etc all help.

This where the better Indians score better, and the arrow hardly matters at all.
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Old November 24, 2012, 08:36 AM   #11
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Quote:
It is not unheard of to get no wind, and right, and left, all at the same time. Happens more at longer ranges, of course (I am thinking 600 yards). Or just one short gust that only affects the bullet for a short distance.
If you have a short gust that deflects the bullet, the bullet will still keep drifting off course after it passes through the gust. The deflected bullet has effectively been re-aimed in a new direction.
That means that a short gust 25 yards in front of the muzzle does a lot more damage than a short gust 25 yards in front of the target.
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Old November 24, 2012, 08:46 AM   #12
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Here's an excellent source on .22 rimfire bullet's ballistics:

http://www.exteriorballistics.com/eb.../22rimfire.cfm

A given bullet will have different BC's for different speeds through the air. Note the three BC's for a given rimfire bullet; one for each velocity range. To get near perfect wind drift, you'll need software that uses several velocity bands' BC values. Sierra Bullets' software may be the only one that does. Berger's software does not. If you check out Sierra's web site for their .224 dia. 90 gr. HPBT Match KIng and you'll see 5 BC's for 5 velocity bands:

.504 @2200 fps and above
.511 between 1900 and 2200 fps
.500 between 1750 and 1900 fps
.467 between 1575 and 1750 fps
.400 between 1375 and 1575 fps

The only way to get exact BC's for bullets is to measure their time of flight over a short distance at different average velocities for that distance, then compare those numbers to a standard bullet shape. Calculations based on shape, weight and speed are only approximate and sometimes way off.

And the wind isn't perfectly uniform in speed from the ground up. If the wind's 10 mph at 10 feet above ground, it could be 6 to 8 mph 1 foot above ground depending on the terrain and obstructions nearby. Ballistic software typically doesn't account for this; they use the same wind speed for bullets' path even it its trajectory's high point is 13 feet above the line of sight at 600 yards for a 1000 yard target.

Last edited by Bart B.; November 24, 2012 at 09:09 AM.
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Old November 24, 2012, 08:48 AM   #13
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You wouldn't need a steady wind. A bc of .235 and a MV of 3600fps has a flight time of .09sec at 100yds and .2sec at 200yds (rough numbers). Wouldn't take a steady wind just a gust.
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Old November 24, 2012, 08:51 AM   #14
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Quote:
Kraigwy, I'll have check but I believe the hornady wind drift table showed a 1" drift in a 10mph wind with that bc and MV? They don't list other wind speeds, hence the question and I think the closest bc was .240. The edition is pretty old, are you sure about those values?
No, I'm not sure of the numbers. All I did was put the OPs data in Berger's Ballistic Program, without changing the default numbers except changing the wind value from 5 to 10 to 15 MPHs.

The chances of the results coming out the same for every location is slim, but the overall results of drift per MPH would be linear.

But this is on paper, none of this would show up in real life. For example, altitude is different, humidity is different and wind is seldom constant across the range.

Also a perfect lined up (not canted) scope and the shooter not canting the rifle is rare. That's why we have different zeros for different ranges.

As you move out (distance wise) the arc of the bullet is higher, and the wind will be different at the top of the arc compared to eye level.

Too many variables to give a correct answer.

Thats why BC programs are guides. They get you close but nothing beats actual shooting to see what happens.
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Old November 24, 2012, 09:01 AM   #15
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You are so right wrt cant. Something I am working on ...I need a scope with a bubble level reticle :-)
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Old November 24, 2012, 09:06 AM   #16
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Kraig says:
Quote:
Also a perfect lined up (not canted) scope and the shooter not canting the rifle is rare. That's why we have different zeros for different ranges.
For hunting rifles, yes. For competition ones, I disagree; it's easy to level your sights if you have and do the right stuff. But that's another thread.
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Old November 24, 2012, 09:12 AM   #17
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Not all competition is from a bench or bipods. Isn't NRA highpower a 3 position match?
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Old November 24, 2012, 09:25 AM   #18
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Quote:
it's easy to level your sights if you have and do the right stuff.
Of course you can, but how many people do.

How many people here (honestly) can say they check their scopes. Lets say your windage and elevation knobs are suppose to be 1/4 moa clicks.

ARE THEY, and are they at all ranges.

Take a tall target, about 4 feet tall. Set it up at 100 yards. Use a plumbob and draw a line down the center of the target and put the aiming point at the bottom.

Now zero at the aiming point. Move your sights 10 up, shoot again at the aiming point, move 1o more up and shoot again. Keep doing this until you run ouf of target.

Now check to see how far off the vertical line you are. Also check distance between groups. Bet their not constant, bet the groups arn't all on the verticle line.

Do the same thing with a horazonal target, left and right of center. Are they constant????

Everything is different, every scope or iron sight is different. You see this on high price scopes and cheap scopes.

This is another reason you can't trust BC programs, but your actual zeros. This is why I always preach the value of a well kept data book.

Yeah, maybe a bit off topic, but is it really? The OP was asking if a wind correction is linerer. It may be, but will our sights, or our shooting position, show this.

Bryan Litz (Berger Bullet Chief Ballistician) just came out with an excellent book, "ACCURACY AND PRECISION FOR LONG RANGE SHOOTING" that explains this much better then I can.

Litz uses what he calls a WEZ (Weapon Employment Zone) analysis showing the effects of such errors.

I highly recommend this book for anyone serious about their precision shooting. Its an eye openner even for an old has been like me who thought he knew what he was doing.
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Old November 24, 2012, 09:31 AM   #19
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Yes, high power with NRA match rifles is 3 position. But even with service rifles without spirit levels on their front sights like many bolt action match rifles have to keep 'em level, windage zeros for different ranges seldom varied more than 1/2 MOA in all three positions for those shooting the best scores. It takes some good training and practice to hold a rifle pretty consistantly level from shot to shot, but it can be and is done. And spirit leveled front sights have been easily aligned with the rear aperture sights windage arm so the sight moves perfectly horizontal when the sights are leveled; been done since about 1960.

With scopes in competition, they're easy to level quite repeatably from shot to shot. First, twist the front sight's barrel band so it's spirit level matches a level on the rear sights's windage arm. Then clamp the rifle barrel in a vise so the sights are level. Then remove the front and rear sight, moung your scope, loosen the rings and twist it so the horizontal wire's level. Then all the sights for that rifle are set up the same.

I've always used the same windage zero for all ranges from 100 through 1000 yards for the same centerfire cartridge. Whatever spin drift there was, it was too small to be corrected for.

Last edited by Bart B.; November 24, 2012 at 10:40 AM.
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Old November 24, 2012, 09:39 AM   #20
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Quote:
Not all competition is from a bench or bipods. Isn't NRA highpower a 3 position match?
Yes and zeros will be different. My 200 yard standing zero is different then my 200 yard setting zero. My rapid fire zero is different then my slow fire zero.

Shooting from a bipod or bench is going to give you a different zero then shooting prone with a sling, or setting, or kneeling, etc etc.

You may have a perfectly plum scope (zero can't) but as you move out spin dirft does come into play. Most people discount it, but its there, granted its normaly hiden in wind est. errors, but its there. How much depends a lot on the twist rate.

Look at your ballistic program. Take SHOOTER for your Ipod or I phone. Plug in all the numbers, set wind at ZERO and wind direction at 0 or 180 degrees. Now check the windage correction to, lets say 1000 yards. Shooter will give you a correction in this no wind situation.

Now go back to your inputs on your SHOOTER program, don't change anything but the twist direction. Change from a right hand twist to a left hand twist (leave everything else alone). Check your wind drift numbers, they are reversed.

Check the clicks on your windage knob, are they a constant 1/4 min all the way out to 20 min left and right??? Same with elevation clicks.

DATA BOOKS, write everything down.
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Old November 24, 2012, 04:07 PM   #21
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Kraig's right about spin drift. It's not much as he says. For example, a .30-06 with a 172-gr. FMJBT bullet leaving at 2700 fps has about 7 inches at 1000 yards. If you have everything perfect and this is what you get, then set your sights between 1/4 and 1/2 MOA left for a zero and the error for drift will be no more than 3.5 inches at 1000, half that at 600, 1/4th at 300 and so on.

If you can judge cross wind values from zero to 1000 yards across your bullets trajectory path accurate enough to make exact wind corrections with no more than a 1/8 MOA error, then you need to be concerned with spin drift. To date, I don't know of anybody that can do that.

One other thing; you and your shooting stuff has to do no worse at 1000 yards than 3 inches maximum spread all the time or you will never see the effects of spin drift.

Last edited by Bart B.; November 25, 2012 at 08:47 AM.
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Old November 25, 2012, 09:19 AM   #22
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Subsonic bullets are also subject to vertical drift, due to rotational friction against the wind. For instance, say a 10 mph wind at 9:00 will move a .22 LR about an inch right of center. The drop will be approximately 1/5 the horizontal. Conversely, wind from 3:00 will cause the bullet to rise about 1/5 the horizontal.

Wind from other various clock positions will create lesser combinations of drop or rise as well as deflection, but any wind angle from 12:00 to 6:00 will cause the bullet to rise somewhat and from 6:00 to 11:45 or so will cause a drop component. Winds close to 12:00 can cause either rise or drop, unpredictably, so rimfire benchresters try to hold the shot and wait for the wind to change.

Swirling winds on bermed ranges can be hell to rimfire bench folks; been there!
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Old November 25, 2012, 09:50 AM   #23
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I think I'm beginning to understand why the only place I have ever managed one-hole groups @ 50 yards with a .22 is in an indoor range.

I don't see why the vertical deflection from bullet spin in a crosswind is limited to subsonic bullets though.

Maybe that's why those fast twist muzzleloading barrels can't compete with the slow twist barrels in matches, even though they can shoot very accurately in ideal conditions.
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Old November 25, 2012, 09:54 AM   #24
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What have you guys found to be the most reliable way when reading wind (mirage, leaves...) if there are no flags? Do you use any handheld devices or other portables?
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Old November 25, 2012, 10:11 AM   #25
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Quote:
I don't see why the vertical deflection from bullet spin in a crosswind is limited to subsonic bullets though.
Its not, the difference is TOF, or time of flight. The longer the bullet is exposed to the wind, the more the wind affects it.

Quote:
What have you guys found to be the most reliable way when reading wind (mirage, leaves...)
All of the above, but I like mirage the best. What we need to understand, is wind is different at different altitudes. Meaning, if your shooting prone, what the wind is doing at 2-3 feet above the ground is not what the wind is doing at the top of the arc of the bullet path.

To see this, use two range flags. One at the bottom of the flag pole, and one 20 feet in the air. You will see, most of the time the flags are indication different wind velocities. May not even be blowing the same direction.

Now mirage is different, you can focus your scope to read the mirage at any place you want to, ground level, 20 ft in the air, across a canyon or where ever (just make sure the scope is NOT focused past the target, that will give you a false reading).

Mirage is not hard to learn, just take practice. Take your wind meter and spotting scope out to the field (don't have to be a range, could be your back yard, a parking lot or what ever).

Look in the scope and observe what the wind is doing. Now take a wind reading. With practice you can get pretty good and accurate.

Same thing with leaves, grass, and trees. Take your wind meter with you on your walks. See what the wind is doing to the leaves, grass, trees, whatever, and check your wind meter to see how much wind it takes to make the leaves, grass & trees do what they do.
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