One of the more common bits of advice to shooters is “Allow a little for the wind.” Most of us know the wind will blow a bullet off course. Most of us don’t know how, why or how far. Any of us planning on shooting much beyond 200 yards should. Wind can turn a hit into a miss, easily shifting a projectile off course by more than a foot at 300 yards.
It’s Really Wind Deflection
The atmosphere is much like a giant ocean with various currents flowing in many directions. Air, in fact, is like water in that it has mass and exerts pressure. Turn a bullet loose in this atmosphere and, even though it is moving rapidly, it is slowed because it has to push (buck) air out of its way.
Bullets also get deflected off course if air currents are flowing at any angle other than parallel to their line of flight. This is in agreement with Sir Isaac Newton’s first law of mechanics that predicts an object will move at a steady speed in a straight line unless acted on by an outside force. Air is that force.
Common sense suggests that a light bullet would be blown farther off course than a heavier one, just as a balloon would be pushed farther than a bowling ball. However, according to the wind drift chart in the “Hornady Handbook of Cartridge Reloading, Fifth Edition,” a tiny 55-grain .224 bullet with a ballistic coefficient of .270 launched at 3,000 fps will deflect 16.3 inches at 400 yards in a right-angle, 10-mph wind. Would you be willing to bet that a 150-grain .308 bullet could beat that? Don’t. According to the Hornady chart, a 150-grain round-nose .308 bullet with a BC of .270, sent flying at the same 3,000 fps as the .22 bullet, will also drift 16.3 inches.
What’s going on here?
Apparently sheer mass isn’t the be all and end all when it comes to bucking the wind. Ballistic efficiency (a combination of shape and weight) and velocity are. If you want to bet on a 150-grain .308 bullet, put your money on a spire point instead of a roundnose. That sharp-tipped slug, thanks to its higher BC of .423, will drift just 9.5 inches at 400 yards (3,000 fps muzzle velocity), handily beating the .22 pill.
Like Water Under a Bridge
One way to picture what’s going on is to imagine pushing a chunk of 2-by-4 lumber across a flowing stream. Point the narrow end at the opposite bank, shove the lumber and watch it drift downstream as it plows across. Of course, if you gave it sufficient energy, it will land, but farther downstream than you aimed. Now, repeat everything exactly, except push it broadside first. Different outcome, eh?
Next, sharpen one end and push with that end forward. Similar things are happening with an inefficient round-nose bullet compared to a sleeker spire point. Sharp-nosed, long, ballistically efficient bullets don’t “buck” the wind as much they slip by it. By minimizing frontal exposure and hiding mass behind a wind-cutting nose, a high-BC projectile maintains more velocity than a less-efficient slug, reaching the target before air currents move it very far.
And unlike the water carrying the 2x4 downstream, wind does not just “carry” a bullet. It actually changes its direction slightly and constantly. The bullet isn’t just drifting with the wind; it’s deflected, actually given a slight velocity in a new direction, and this velocity thus carries it farther off the line of sight. It’s almost as if the bullet ricochets off the wind.
Once wind has deflected a bullet, it continues that sideways momentum — a new angle of flight. In other words, if a 20-mph wind over the first 100 yards of bullet flight pushes (accelerates) it left at, say, 2 inches at 100 yards, and then suddenly stops, the bullet will continue shifting left until air drag eventually stops it. Constant wind pressure accelerates it more. It’s Mr. Newton’s first law of mechanics again.
One problem with all this is that the mathematically computed wind drift charts rarely seem to apply accurately in the field. This is because nature isn’t a controlled experiment. We cannot guarantee wind speed and direction. Is it 10 mph, 13 or 7? Is it quiet near us, but gusting downrange? Is it coming from a right angle or a quartering angle, at which it has but half its value?
What’s a shooter to do? Your best defense is to employ the highest BC bullet your rifle will shoot accurately at the highest velocity your cartridge/rifle will handle safely. It also means some small-caliber bullets can outperform some larger calibers. The .257 Weatherby Mag and .264 Win Mag, for instance, can be loaded to buck the wind better than some .300 magnums. This means you don’t have to put up with the recoil of a 180-grain bullet at 3,100 fps to hunt pronghorns or mule deer.
Target shooters learn to use wind flags or watch heat shimmer to judge wind direction and velocity. Holding “half-a-dog” into the wind is a familiar but unscientific technique employed by experienced varmint shooters. It doesn’t have to be scientific because one can compensate on the fly. If the dust is observed erupting to the left, one holds farther right. At this point, some like to add a few clicks to their scopes, some prefer mil-dot or multiplex reticles with wind-compensation posts, and others do just fine holding the crosshairs off the target.
While effective for rodents, this shoot-and-learn technique doesn’t play well with predators or big game, which tend to decamp after the first near miss. In this case, it behooves you to know how your rifle and bullet perform in various levels of wind so you can compensate as necessary.
Of course, this can also get you into trouble. Back in the mid-1980s when I was beginning to study wind drift and memorize its effects, I missed an easy coyote by holding just off hair into a stiff Kansas breeze (which would have been reported as a gale in my home state of Indiana.) According to my memorized wind-drift tables, my little 50-grain Hornady bullet should have landed 7.897 inches right of the point of aim. It ended up raising dust right about where the crosshairs were when the trigger broke.
The truth is, judging wind and range are more art than science, and overplaying it can cost you game. A rule I heard first from pronghorn guide Keith Atcheson of Atcheson and Sons Hunting Consultants in Butte, Mont., was “Never hold off hair on your first shot.”
Recently I targeted a Coues’ whitetail 350 yards across a canyon, aiming just behind the shoulder with a 7mm Rem Mag rifle. At the shot, the deer collapsed, punched through the neck. An unfelt wind blowing down the canyon between us had deflected that speedy 150-grain spire point more than a foot.
Given incidents like these, perhaps an effective compromise would be to factor the wind drift, but cut it in half for your first shot. Better yet, cut the range in half before you shoot. Wind deflection beyond 150 yards is much less than it is beyond 300 yards. By all means, range-test your rifle/bullet on paper in big winds and compare the results to computed charts. Most bullet makers rate their products’ BCs optimistically. Also, individual rifles stabilize bullets to varying degrees.
Bullet drift and deflection isn’t voodoo, but there are enough variables to make it highly unpredictable.
Subscribe today to have GunHunter delivered to your home.