What's the real deal with wind drift?

Dave R

New member
I hope some of you physics types, or maybe some of you experienced shooters, can help
me figure out the physics of wind drift. I had a discussion with some folks at the range
the other day that left me confused.

I have access to two ranges--an indoor range with 25 yds. max, and an outdoor range that
is frequently windy. Very windy. So when I got my Rem 700 in .308, I sighted it in at
the indoor range at 25 yds (after dark on a work night), then hit the outdoor range
Saturday to sight in at 100yds.

It was blowing. The flags at the range were about straight out, with the wind exactly
crosswise to the range. Blowing from right to left. I estimate the wind at a steady 10-12 mph. I was shooting at a sight-in target, with 5 bullseyes. I was lucky I shot my first group at the upper right bullseye, ‘cause the group wound up near the upper left bullseye. That’s over a foot left of where I was aiming.

Now I didn’t want to sight my scope to the right bulleye, and then find out that, without that wind, I’d wind up a foot RIGHT of where I wanted to be when there was no wind. So I shot another group at the lower right bullseye, and that group wound up near the lower left bullseye. The groups were just over an inch, so I was happy, considering the wind. But when I asked the range boss how much I should expect the wind to move my point of impact, he said it should only move it about an inch. Now remember, I’d sighted in the night before at 25yds, and I found it hard to believe that I could be that far off, or that a wind that strong would only move my POI an inch.

Well, I did some math, to try to figure what that wind would do to POI. Here are my
calcs.

If the wind is 10 mph, I’ll convert to FPS to make the next step easier. 10mph times 5,280 for feet gives me 52,800 FPH (feet per hour)

Divide by 60 to get feet per minute gives me 880 FPM

Divide by 60 again to get FPS and I get 14.7FPS wind speed.

Now, how long is the bullet in flight, to be acted upon by that wind? My factory 165gr
round is rated at 2700fps muzzle, 2,500fps at 100yds. So the average velocity over the
whole trip is 2,600fps. So, to travel 300 feet at 2,600fps means 300/2600 or .115
seconds.

Winds at 14fps for .115 seconds should move the bullet 1.7 feet, or about 20.4 inches.

Right?

Well, the discussion that ensued between me, the range boss and several other shooters
got pretty lively. I will summarize.

One view: MOVING MEDIUM The bullet is traveling through a medium that is moving
(the air) so the air will move the bullet as much as it moves. i.e. 20.4 inches. That’s a bit more movement than I was seeing on the targets, but not a lot. Same as if a speedboat is crossing a current. The captain can predict his arrival point by calculating how much the current will move his boat in a specified time.

Another view: INERTIA. That bullet wants to move in a straight line. The wind wants
to move it. But the bullet’s inertia prevents the wind from moving it as much as the math would say, so the POI is closer to aim than the math would say. Hmmmm.
Makes some sense.

Another view: AERODYNAMICS. The more aerodynamic the round is, the less the
wind will affect it. An aerodynamic car moves less in a crosswind than a boxy car. So wind will move an aerodynamic bullet ( a boattail) less than it would move a boxy bullet. Or is that just the velocity making the time in the air shorter, so the bullet moves less?

Another view: WEIGHT. Some folks said a heavier bullet will move less than a light
bullet, because it has more inertia.

So who’s right? I favor the “moving medium” view, and think the math tells where the
bullet is going to hit. The other factors relate to velocity, which relates to time in the air, which relates to more or less movement. But nobody could agree on what factors were affecting the bullet how much.

Your thoughts, please?
 
To work up a load in a crosswind, don't worry about your horizontal displacement. Look for when you've got a string of shots that are close together vertically... THEN, start paying attention to how the wind is blowing, and shoot only when it's blowing in one particular way.

A mild little wind can blow a .25" group out to .50" easy...
 
The fallacy in your logic is your premise that your sighting your rifle in at 25 yds should put you close to zero at 100 yds. Better do some more signtin' n shootin'. A 15mph crosswind won't blow a .308 projectile very far in 100 yds. The theory that the projectile moves the same distance crosswise as does the wind in the same time is sophomorically silly.
 
In the real world I haven't noticed a huge movement with the .308 at 100 yards, even using 100 grain plinkers. However my 165 grain HPBT's were blown 14 inches off by about a 15 mph crosswind at 300 yards.
 
WalterGAII and KilgorII illustrate some of the discussion I was getting at the range.

WalterGAII, I was real close vertically--why over a foot off horizontally? KilgorII's experience is similar to mine--with similar bullets. Can you offer anything more explanatory than "sophomorically silly"?
 
The this is, is that all the things listed, moving media, inertia, aerodynamics, and weight all play a part, not just one thing. Now I'm just an armchair quarterback in all of this, but heavier bullets, moving faster, with a better BC (ballistic coefficient-ie aerodynamics) will fly straighter than a light, slow moving, high BC bullet (light bullets lose their speed more quickly than heavy ones, and that only adds to the complexity of it all.) The .50 BMG is an excellent round to study, as it's BC is close to perfect, (depending on bullet type) and it "bucks" wind like mad, relying on Weight, more or less constant speed (which is a by-product of the weight and goo BC) and a good BC. Compare this to, say the .300 Win Mag shooting Speer Plinkers.

Just recently here, someone here did a study on the ballistics of the .50 BMG vs. a .300 WM with a real bullet (vs. the Plinker) At about 700 yds, the .50 BMG passes the .300 WM. It keeps on trucking with relatevely no loss in speed compared to the .300WM.

I have no idea where I'm headed with this, other than to bring up points in this already perplexing area of shooting. I must learn more!
 
Thanks, Frontsite! Maybe the reason I'm getting so much controversy is that it IS a complex issue...all variables interact?

BTW I finally skimmed the link in the first post. Didn't read it all but what I read seemed to support the "bullet moves as much as he wind" theory. When I actually read it I'll post a summary.

As FUD says, share what you know, ask what you don't.
 
The reason why it's complicated is that the drift can only be calculated by knowing the force exerted on the bullet. Force is a function of wind speed, air temperature, air density, wind direction, projectile configuration, projectile mass and projectile speed (time to target). I think that's all the factors. But it's enough to warrant use of tables specific to your bullet.
Rich
 
Go to the place suggested by tuc22:
"...we can now use the standard formula for finding wind deflection (no it’s not RxV over 10!) which can be found in the 1929 British Textbook of Small Arms:
D= (x-x’) xW
In this formula, W is the windspeed in feet per second, D is the wind deflection in feet, x is the time of flight in air (from the table above), and x’ is something called the vacuum time of flight or VTOF. To find the VTOF, take the previously mentioned muzzle velocity and divide it into the distance covered in feet. By the way, the value of x-x’ is also known as the delay (due to the delay caused by deceleration due to air
resistance)"

What this means is that the bullet is only deflected in direct relationship to the loss of velocity it suffers in actual flight. Zero loss of velocity (impossible, by the way) = zero deflection; great loss of velocity = lots of deflection.

The loss of velocity is mostly determined by the shape and weight of the bullet (both determine the coefficient of drag), and to a lesser degree the elevation, humidity, and temperature (ergo, density) of the air. The coefficient of drag also varies with the velocity in relation to the air, is constantly changing and normally is separated in three or more "zones" with similar values.

After doing the example you can check a ballistic program and find that deflection for a 10 mph 90º wind is indeed a bit less than 1".
 
Dave:

"Sophomorically silly" was just the best alliteration that I could come up with at the moment. Those guys over on benchrest.com could quote you chapter and verse with just about any caliber and bullet design. For anyone to believe that a fired bullet moves the same distance in windage as does the prevailing wind at the time, in the same time, is beyond my comprehension. It's just a little more complex than that, as has been adroitly pointed out by a couple of other posters.
 
DaveR
All I can tell you is to start doing some long range shooting at whatever range you can get to. If the range is only 100 yds try it with a 22 rimfire on a windy "cross" day.
IMHO, Wind is a personal thing and it is a bitc!
 
As mentioned elsewhere, tables for this sort of thing exist. Highpower rifle shooters deal with this sort of thing all the time out to 1000 yards, and reading wind and making the necessary corrections is critical. Fortunately you are working with .30 caliber and there are literally decades of experience and data on hand.

Case in point, though: Today I was firing on the Army 'D' tgt at 500 yds with an estimated 15 mph wind from the right. 15 mph X 5(00) yds divided by bullet drift factor of 11 (for Sierra .30 cal 168 gr. match bullet) yields a figure of 6.81; that translates into 7 'minutes' (a minute being 1" at 100 yds, 5" at 500 yds) correction into the wind. In other words, the bullet was being pushed 25" over. I called it right and my 3 shot group was just outside the V ring. I have fired in winds up to 20 mph which gets to be quite a challenge. Even when you have a known distance target reading wind is the big problem. At 100 yds, of course, the drift would be 6.81" for that bullet. Velocity and time of flight, etc., are factors, but as you can guess the variables don't hold still, either.

'Reading wind' can be done by simple estimation (not so simple), watching flags, reading mirage, and I am tempted to get a wind meter to improve my estimates.

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I read somewhere, I think it was Ultimate Sniper by Plaster, something to the effect that"... a novice shooter reads trajectory tables and an expert reads the wind." Everyone judges the wind differently. As long as you judge it consistantly, practice will provide you with proper "Dope" for your rifle/scope, and proper record keping will allow you to reproduce results.

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Mark
 
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