June 03, 2011
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.
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.
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.
"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.
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.
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.
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.
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.