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Don’t Blow A Shot In The Wind

by Scott E. Mayer   |  June 3rd, 2011 14

Wind does weird things to the flight of a bullet, but more often than not, you’ll miss in the wind because of what you think the wind is doing to the bullet.

Wind does weird things to bullets, but one it doesn't do is "blow" them off course. Understand how wind "deflects" bullets, and be a better shooter.

I pushed my cowboy hat back a little to clear the ocular bell of the scope and wedged it down tight to my head so the stiff crosswind wouldn’t snatch it off again. I peered through the scope at a fat prairie dog sitting on its mound and that was separated from me by about 200 yards of unusually lush Kansas grassland. It should have been an easy shot, but mirage played its usual tricks and it occurred to me that even though I had sighted it in, I really didn’t have a clue where the borrowed 22-250-caliber Kimber rifle would shoot in this wind. I had a vague idea of the wind speed and the deflection. It felt to me like the wind should push the little 50-grain pill off by about double the width of a dog, so I held into the wind that much to the left for the shot. As usual, the real world didn’t much care how I felt. The bullet hit the mound left of the dog, showering it in a curtain of moist dirt, still weighted from the previous night’s rain.

That wasn’t the first time the wind had done me in afield. I can’t think of anyone who can legitimately call themself a deer hunter who, like me, hasn’t had a cagey old buck or, more likely, an unseen doe, catch their scent carried on a changing wind and alert the entire woods to their unwelcome presence. Target shooters, too, have to contend with the blow-hard gremlin, but they have the benefit of known target distances, the use of wind flags or wind meters, and published wind deflection tables at their fingertips to keep them from dropping a point.

As I believe is the case with most folks, I overestimated the wind speed when I held off on that prairie dog, and missed because of it. I decided at that moment to come up with some real world examples of wind speed, measure them, and commit them to memory for future reference.

When shooting prairie dogs, knowing how much to hold off for the wind means the difference between a hit, and a 'dog getting a dirt shower.

My cowboy hat was just able to stay on during the prairie dog shoot where I later measured the wind speed at 10 mph. To give you an idea of what a 10 mph wind feels like, get a big floor fan, turn it on high, and sit right in front of it a few inches away. You’ll probably have to squint your eyes a little, and even then they may water some, but that’s a 10-mph wind. And for the 22-250 with 50-grain Winchester Ballistic Silvertip factory ammunition like I was using, the wind deflection at 200 yards for a 10 mph crosswind from 90 degrees was more in the neighborhood of 3 1/2 inches, not the six to eight inches I held for. Had the crosswind been more like 20 mph, where the deflection for that load is a little more than seven inches, I wouldn’t have missed. I probably wouldn’t have taken the shot in a 20-mph wind.

In a 20-mph wind, you better have a baseball hat on tight, or it’s gone. If you have a small desk fan, it will give you a breeze between five and seven mph. So you see, it’s easy to think that the stiff breeze trying to lift your lid can deflect your bullet more than it really does. Unless you’re shooting a really low-powered rifle cartridge such as a 30-30 Winchester, or at long range of 250 yards or more, it pays to err on the side of a little less hold off for the wind.

Some folks like to know the deflection for each one-mph of wind, and that’s fine if it works for them. For me, I find it simpler to memorize deflection for 10 mph because deflection is proportional to wind velocity, so if the wind is blowing half that—5 mph—then deflection is half what it would be for the 10 mph wind.

Many shooters mistakenly believe that the wind “blows” the bullet off course, and that the amount it deviates off course is directly dependent on the bullet’s time of flight and its cross-longitudinal area. In other words, the amount of bullet exposed to the wind, and the duration of that exposure.  All that sounds reasonable, because we can see how the wind blows tumbleweeds across the prairie or leaves from a tree in the direction the wind is blowing, so it makes sense that the wind similarly blows the bullets off course in the direction the wind is blowing. It also makes sense that the longer the bullet is exposed to the crosswind, the more the crosswind moves the bullet. It further makes sense that the larger a bullet, the more area there is for the wind to affect, and therefore the more the area the more the deflection.

Well, as is the case with most ballistic observations, it’s not that simple.

A real-world way to conceptualize that the bullet isn’t simply “blown” off course by the wind was explained to me by my late friend and noted ballistics expert William C. Davis, Jr. Mr. Davis offered the following as an experiment.

A .22 Long Rifle bullet will drop 14 inches in the same amount of time it take it to go 100 yards. In a 10 mph cross wind, the bullet deflects 5.2 inches. The simple experiment of dropping a bullet in a moderate breeze proves that bullets are not "blown" off course by wind.

“It is a fact that a 22 Long Rifle bullet fired at a muzzle velocity of 1255 fps will reach a 100 yard target about 0.269 second after firing, and the wind deflection in a 10 mph crosswind will be about 5.2 inches. It is also a fact that a 22 Long Rifle bullet simply dropped from the hand at the instant the gun is fired would fall about 14 inches during the same 0.269 second time of flight, and thus it would be exposed to the wind for the same length of time as the bullet that was fired at the 100 yard target.

“If the wind deflection depended directly upon the time that a bullet is exposed to the wind, the 22 Long Rifle bullet simply dropped from a height of 14 inches in a 10 mph crosswind would ‘drift’ sideways about 5.2 inches during its fall. We probably know intuitively that this will not be the case, but if experimental proof is needed, then the simple experience of dropping a 22-caliber bullet from a height of 14 inches in a moderate breeze will be enlightening.”

“Drift” is the deviation of a bullet to the right for rifles having right-hand twist, or to the left for rifles having left-hand twist. It’s caused by the interaction of the axial spin and the aerodynamic overturning moment, which causes the nose of the bullet to point very slightly to the right or left, respectively, of the trajectory. Drift is, for our purposes, independent of wind deflection, though its value and wind deflection are additive. That is, if drift is to the right one inch at a given range and wind deflection is to the right five inches at that same range, then total deviation is six inches right (1+5=6). Likewise, if wind deflection were to the left five inches in the above example, then total deviation would be four inches to the left (1-5=-4).

Wind “deflects” a bullet. Look up “deflect” in the dictionary and it is defined as “to turn aside or cause to turn aside; swerve.” And that is exactly what the bullet does, it turns in the direction of the wind. If you plot the path of a bullet deflected by a steady wind, you’ll see that its path is curved. If the wind were directly blowing the bullet off course, the deviation would be in a straight line at an angle from the line of sight.

When we look at wind deflection, we have to understand that the bullet is taking a new line of flight because of the deflection. And because of that, the wind most deflective to a bullet’s flight is at the muzzle, not downrange where the bullet is moving slower. To visualize that concept, think of the line of sight at the instant a gun is fired as a ray, and of the path of the bullet as another ray, with the rays both originating at the muzzle so that they form an angle. If an instantaneous wind deflects the bullet on a new line of flight the moment it leaves the muzzle, you can visualize that if the bullet stays straight on its new path, as the range increases, so does the distance between the rays. Now if the wind is steady across the entire range, it is constantly updating by deflection the bullet’s line of flight resulting in the curved bullet path.

A bullet deflected by a steady wind will show a curved path (l.). If wind directly blew a bullet off course, it's path would be straight (c.). With wind deflection, the bullet is constantly taking a new line of flight. In a constant crosswind, the bullets new line of flight is being constantly updated resulting in the curved path we know as deflection. (r.).

To minimize the effect of wind, we don’t necessarily minimize the bullet’s time of flight by giving it higher velocity, but rather we minimize the bullet’s lag time. Lag time is the difference between the time it would take a bullet to travel a given distance in a vacuum compared to its time of flight under actual conditions.

The formula for wind deflection is D=W(T-R/V) where D = bullet deflection in feet, W = crosswind in fps, T= time of flight in seconds, and R/V = range in feet divided by muzzle velocity in fps. If we launch a Sierra 190-grain MatchKing bullet from a 300 Win Mag at 2,900 fps, it would take the bullet 0.107 second to reach a target 100 yards away where its velocity would be 2,725 fps. In a vacuum, velocity would stay 2,900 fps and it would take 0.103 second for the bullet to go 100 yards, so the lag time is 0.0035 second. For a 10 mph, 90-degree crosswind, the deflection is 0.6 inches.

A wind directly from the rear, or tailwind, will reduce time of flight relative to the air in which the bullet if flying. A tailwind does not affect bullet horizontal dispersion enough to matter, unless the tailwind is fishtailing.

If we substitute Sierra’s 168-grain MatchKing bullet, we have to start it at a higher initial velocity—2,925 fps—to retain a 0.107 second time of flight to 100 yards where its velocity will have dropped to 2,706 fps. In a vacuum, velocity would stay 2,925 fps and the bullet would take 0.102 second to go 100 yards, so the lag time is 0.0044 second. For a 10 mph, 90-degree crosswind, the deflection is 0.8 inches. You can see by that example that both bullets had the same time of flight. The 168-grain bullet with its smaller cross-longitudinal area and greater initial velocity had more deflection than the larger, “slower” 190-grain bullet. The difference, of course, is that the 190-grainer didn’t lose its velocity as quickly, and thus had less lag time.

Very rarely will a wind be exactly from 90 degrees. A wind from one, five, seven or eleven o'clock is given half the value of a 90 degree wind. Winds from two, four, eight or ten o'clock are given almost full value (0.87) of a 90 degree crosswind.

We can reduce wind deflection by improving the efficiency of the bullet’s use of velocity, and by that I mean its ability to overcome air resistance and maintain its velocity. We get that by increasing ballistic coefficient, which results in less lag time. It is also true in many cases that deflection at a given range is decreased by simply increasing velocity, though at transonic and subsonic velocities that is not necessarily the case.

Occasionally, a shooter will be blessed with a perfectly calm day when the wind flags don't so much as ripple. But when the wind is blowing, it pays to know what it will do to your bullet.

Another important thing to realize about wind and deflection is that not all winds are from 90 degrees, so it is important to know not only the wind speed, but also the direction. Wind downrange can be blowing in a different, even the opposite direction from a wind at the shooting bench, and the angle of the wind affects the amount of deflection. For winds that are not 90 degrees from the line of fire, it is the sine of the angle between the line of fire and the direction of the wind that gives you the effective crosswind. If you view the range as the face of a clock with the target being at 12 o’clock, then a wind from one, five, seven or eleven o’clock is given half the value of a 90 degree (three or nine o’clock) wind. Likewise, winds from two, four, eight or ten o’clock are given almost 90 percent (0.87) the value of a true 90 degree crosswind. For example, if a 10-mph wind is coming from eleven o’clock, its value is half, so correct for a 5-mph, 90-degree crosswind. If the 10-mph wind is from four o’clock, its value is .87, so hold as if the 90-degree crosswind was 8.7 mph. The tricky part is if you have a head or tail wind that changes from slightly left to slightly right. For our purposes, a true head- or tailwind does not affect horizontal deflection. But if, for example, a headwind changes from 11:30 to 12:30, the deflection goes from being “X” inches right to “X” inches left. If a wind is from 11:30 and a shooter corrected for “X” inches right and fired at the moment the wind changed that slight amount to 12:30, the miss will be by 2X to the left.

I know it all sounds very confusing, which is why I think it’s important for shooters to find certain benchmarks for their gun and load if they think they may have to take a long range shot. For the load you use, know the deflection of your bullet at a certain distance for either a one-mph or a 10-mph, 90-degree crosswind. Understand that if the wind comes from a sharp angle either toward or from the target, the deflection is going to be about half, and if the wind comes from closer to 90 degrees, the deflection will be almost what it is for 90 degrees. But most of all, know that the wind usually seems to be blowing harder than it really is, and that the deflection probably isn’t as much as you think. Err on the side of less wind, and you won’t blow it.

  • Russ

    Great article! Now, has anyone put together a table of deflection/correction for some of the more popular/common calibers,i.e. the .22, .223/5.56 or .30-06?

  • AL

    I'm reminded of a line from the movie "Pirates of the Caribbean" spoken by Cpt. Barbossa, "those be a lot of long words missy, and we ain't but humble pirates…"

    Does it really take over 2500 words to tell us that wind usually feels stronger than it really is and that wind deflection usually isn't as severe as most expect?

  • http://none Mark Vanderhoof

    I shoot a 25-06 & never have had a problem with wind, I have made very long shots in very strong winds. Never held into the wind. My load is 52gr. 4831, 120gr. sierra Bt. Hp. About 3,000 ft. The shots were between 250-300 yrds. M.V.

  • gil

    At the velocities quoted, one must consider the air to be a liquid rather than a gas, and as a result, there is greater deflection. think of a boat in the water with various eddies and currents.

    • Scott Mayer

      No. Please DON'T think of a boat on the water. That is not what is going on with wind deflection. Very good read in the Sierra handloading manual about this. More than I am going to follow up with in a comment.

  • HOWIE CLARK

    GREAT SIMPLE EXPLANATION OF THE DEVIL. LOVE IT AND PLEASE KEEP RIGHTING ON HOW TO TAME IT.

  • Roger Whipple

    This is a very interesting article and provoked a lot of thought as I was one of those who believed a bullet was simply blown off course. But your experiment has convinced me otherwise. I did not, however, run your numbers to check your math.

    Although you did not fully explain the mechanism for deflection I see your argument that the bullet tends to "fly" off course through asymetrical drag forces because it does not fly normal to the moving air mass.

    One thing I take exception to though is your statement that "If wind directly blew a bullet off course it's path would be straight." If the bullet locked onto the air mass – like a boat rowing across a flowing river – this would be true. But, it does not ever catch up to the air flow. It would be accelerated by the drag force inparted by the cross flow. Because it would be accelerating in the direction of the side force it would be picking up speed in that direction making a curved path also. Granted, the arc (and change in POI) would be far less than the actual situation as the fan/drop experiment demonstrates.

    • Scott Mayer

      Roger, you are so almost there, but purge that boat example from your mind. Instead, go to the toy store and buy a gyroscope. Play with that, particularly with displacing the tip of it from vertical and I think you'll get there.

      • Roger Whipple

        Scott,

        Since I live in MA I can only dream of shooting at prairie dogs at 200 yds. So, much of my enjoyment of the shooting sports is in the scientific world of ballistics. In the 60's I began my engineering career in the Fleet Balistic Missle Lab of the MIT Draper Labs working on guidance systems. I got a pretty good understanding of gyros and precession. I'm aware that the axis of rotation tries to move at 90 deg to any applied force. The forces in this case are pretty complex.

        As a visual example, a football in flight only momentarily has its spin axis aligned with its balistic trajectory, and neither does the bullet. I am now anxious to read Brian Litz's book on the subject.

        My example of the boat in water still holds as what is NOT going on here. Again, thank you for the thought provoking article.

  • bulletbill

    Perhaps you should give Bran Litz, at the very least, a citation since it looks like the information came from his book.

    • Scott Mayer

      This is an article I originally published in Shooting Times magazine June 2004. The Litz book came out in 2009–five years later.

  • bulletbill

    Well then perhaps Mr Litz should provide a citation to your work in his book as they're extremely similar.

    • Scott Mayer

      LOL! No, I think considering his aerospace engineering degree, work in the Air Force on air-to-air missile design, and position as chief ballistician for Berger Bullets we can assume he has a well grounded understanding of external ballistics. I have not read his book, but now plan to. Any similarity between my article and his book we'll just consider a mutual understanding of wind deflection and coincidence.

  • http://first-edition-online.com Gene Williams

    I have been firing at PA groundhogs and now MT prairie dogs and gophers for 40 years or more. The best teacher I had regarding wind drift was my Red Ryder BB gun. You could actually see the BB curve onto the target if the light was right.

    I love the 220 Swift. I also like the 22/250 a lot. I have missed my target at extended ranges (200 yds or more) with both.

    This is an interesting article but as I read it I think it is providing technical knowledge that may be more than I will ever need. I could be wrong, but I have held into the wind and head shot truckloads of groundhogs out beyond 200 yds.

    The best way to judge you wind estimation ability is to try to put that fifth rd. into a 4 round cluster that looks like a one holer.

    I don't see the significance of whether a bullet is curving or being blown. If you estimate where your bullet will go, based on your experience firing your weapon and lots of shooting hours under your belt, you will score. If not you won't.

    Please do not interpret this as criticism of the author. I don't think that you can know too much about shooting unless some of what you know has a negative impact on your ability to hit the target.

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