The coyote came across a broad flat, orange in the early New Mexico sun. It loped easily, head swiveling, its near-white coat rippling in a brisk wind. The wind cut noiselessly across the muzzle of my 6.5 Creedmoor from 10 o’clock.
“Come closer,” I whispered.
But at just over 200 yards the coyote pulled up, suspicious, facing me. Slinged up in a sit, it was all I could do to keep bullets in softball-size targets at 200. This breeze would move my shot to the edge of that zone. Tugging the sling high to add tension, I nudged the quivering crosswire to shoulder’s edge and fired. The coyote collapsed.
Bullets and the Wind
Bullets never travel straight. In still air, gravity takes them in long, parabolic arcs. When that air moves, it pushes the bullet sideways. Alas, unlike gravity, wind force and direction are not constant. In competition, I’ve fired through wind that straightened target flags one way while swinging wind vanes on the line in the opposite direction.
A bullet sent through that gauntlet would, literally, fly a ziz-zag course. While wind at the muzzle has more effect on impact point than does an equivalent shift downrange, the bullet is more easily moved the farther it gets from the muzzle, because downrange there’s more time per unit of distance traveled for the wind to work its mayhem.
Wind speed and angle matter. “Full-value” wind from 3 or 9 o’clock has the greatest effect because, like gravity acting on a bullet fired horizontally, it is pushing at right angles to the bullet’s path. Wind from 12 or 6 o’clock has essentially no effect unless it is very stiff, and then the result might not be what you think. A bullet fired at a distant target across level ground is actually launched slightly nose-up and remains nose-up.
Surface exposure due to the bullet’s attitude affects its path — though insignificantly at hunting ranges. Unlike an arrow, a bullet is not heavy at the front and does not “porpoise.” So you can ignore most headwinds and tailwinds. Remember that a bullet moving 3,000 fps bucks terrific resistance even in still air. It is, in fact, generating its own 2,000-mph headwind! What difference do you think a 20-mph headwind or tailwind will have on this bullet’s flight?
Bullet Design and the Wind
Vulnerability to wind also depends on a bullet’s speed and its ballistic coefficient, or “C.” Bullets of similar C, driven at the same speed, yield about the same drift. Consider a quartet of pointed bullets with Cs of .390 to .410: a 100-grain .243, a 130-grain .270, a 140-grain 7mm and a 165-grain .308. Pushed at 3,000 fps, all drift about 6 inches at 200 yards in 20-mph crosswind. Drop C from .390 to .290, with, say, a 150-grain .308 protected-point bullet, and drift goes up 50 percent, to 9 inches! Throttle the bullets, and drift increases as velocity drops.
My introduction to wind came in small-bore prone matches. When I moved outside from indoor three-position shooting, I felt as though I’d been plucked from a hotel pool and dropped into the North Atlantic. Savvy shooters learned prevailing winds on specific ranges and the net effects of contradicting flags.
The Spokane rifle range, where I often competed, is on a river bank. Wind typically angles across the firing line from 7 or 8 o’clock, bounces off the bank and hits the targets from 4 o’clock. If you minded only wind at the line, you’d make a mistake. But if you waited for still conditions, you’d run out of time!
Wind, Drift and Bullets
When hunting predators, you’re not using a .22 rimfire to nip X-rings the size of a dime. Bullets from most centerfire rifles can drill moderate breeze without significant deflection inside 150 yards. But at long range, even sluggish air can make you miss. A few rules of thumb can help, provided you can read wind accurately. Remember that effective wind speed incorporates angle. A 10 mph right-angle push becomes a smaller problem as the wind shifts to 10 or 11 o’clock.
Double the wind speed and you double the drift. Halve the wind speed and you halve the drift.
Reduce the wind’s angle from 90 degrees and you reduce drift proportionately. Change the shot distance, however, and the drift might surprise you.
For example, a 130-grain .270 bullet launched at 3,000 fps drifts less than an inch at 100 yards in a 10-mph wind. At 200 yards, it is 3 inches off course — four times as far as at 100! At 300 yards it drifts 7 inches, at 400, 13. There’s negligible drift at 100 yards, partly because the bullet arrives in just 1/10 second. There’s not much drop at 100 either.
Adding distance dramatically increases drift. In fact, drift for the .270 bullet at 500 yards is roughly 60 percent greater than at 400. For many popular centerfire rifle loads, a handy rule of thumb is to assume an inch of drift at 100 yards, then double that at 200. Triple the 200 drift at 300 and double the 300 drift at 400. For example:
Drift of an 85-grain .243 (Federal) Trophy Copper bullet driven at 3,200 fps in 10mph right-angle wind:
Actual drift (inches) Rule of thumb drift (inches)
100 yards 0.7 1
200 yards 2.9 2
300 yards 6.8 6
400 yards 12.5 12
In this case each estimate is within an inch of actual drift. Nobody I know can hold within an inch at 400 yards under field conditions, and darn few rifles will shoot even half that tight. If you’re shooting a .22 Hornet with a blunt 45-grain bullet, the rule of thumb fails beyond 100 yards because the missile is slow and decelerating at a high rate. By the way, very lightweight bullets driven fast can show significant drift at distance because their low sectional density reduces C values. They, too, decelerate quickly. The 70-grain .243 Nosler Ballistic Tip has a C of .276; even when driven at a blistering 3,850 fps, it drifts 15 inches at 400 steps! A lightweight small-caliber bullet can thus buck wind as well as or better than heavier bullets from a bigger bore. For example:
Range (yards) 0 100 200 300 400 500
.30-06, 110-grain velocity (fps) 3,330 1,240
drift (inches) 1 6 15 30 52
.223 Rem., 55-grain velocity (fps) 3,240 1,270
drift (inches) 1 6 15 29 50
Bullet Design, Part II
Sharp polymer bullet tips are all the rage now, but it’s easy to lean too heavily on a sleek nose to win battles with wind. A ballistician told me long ago that the profile of the first 1/10th inch of a bullet’s nose doesn’t affect drift. The ogive, or leading curve of the bullet between tip and shank, matters more.
Boattail bullets become an asset only at very long ranges or in gale-force winds. A 30mph wind that shoves a flat-base 7mm bullet 17 inches at 350 yards moves a boattail bullet 15 1/2 inches. Reduced drift afforded by a tapered heel at higher wind speeds is academic, given how hard it is to estimate drift in strong wind, or hold within two minutes of angle when your hat is leaving you at speed. Also, percentage difference in wind deflection between flat-base and tapered-heel bullets is about the same for a 10mph wind and a 30mph wind.
What's a Mirage?
Wind doping is an acquired skill. Early in my competitive shooting career, fellow rifleman Dick Nelson (who also helped Boeing engineer the first moon vehicle) took me aside. “Mind the mirage. Learn to read that, and your bullets will hop through the 10-ring like trained pigs.”
Mirage is a visual distortion caused by heat waves rising from the earth’s surface. If you don’t see it, it’s not there. Mirage doesn’t move bullets; its dance shows you wind that does. Mirage also can show you a ghost target, by “floating” the target image in the direction air is moving. You won’t see mirage at all distances at once. You’ll see the strongest mirage or the mirage at the distance for which your scope is focused. To get the most information about wind that most affects their bullets, match shooters typically focus their spotting scopes to read mirage just shy of the targets.
Mirage that’s bumpy and moving slowly indicates light breeze. Flat, fast mirage clues you to stiff wind. When mirage suddenly vanishes with no change in light conditions, it’s often because the wind has picked up. Mirage that boils vertically reflects a still condition — but beware, as a boil commonly precedes a reversal! Many competitive shooters zero for light prevailing wind, then hold their fire during boils and reversals, shading and shooting during pick-ups and let-offs.
On summer’s prairie, mirage can help you hit. Fall hunting seasons bring cold weather, which all but cancels mirage. To read wind then, you must tap other indicators: nodding trees and grass, leaves and snow and mist that yield to wind. You won’t always get it right, but practice reading wind will pull your bullets closer to center.
Moving air affects every shot. Like a spouse, wind can bring you grief when you ignore it!
