Before turning inside out while killing an elk, a bullet endures pretty rough treatment—mainly from hot, high-pressure powder gas and the searing press of rifling. But conditions have their way with bullets too. Wind, gravity, temperature, air density, humidity and shot angle don’t deform the bullet, but they certainly influence its flight path.

So does snow. The elk were lining out across a broad white valley below when my partner reached the brow of the hill. He flopped prone, breathing heavily after the fast climb, ankle-deep. “How far?”

I guessed it, now, at nearly 300. The bulls at the end of the column twined in and out, giving the hunter little opportunity. But now one swung to the side and stopped. The crack of the rifle died suddenly against December’s powder. The elk—all of them—trotted off into the distance.

Our guide spared me the duty of calling a miss. Then we saw it: a long, pencil-thin gash in the soft snow in front of the rifle. “You had a clear line of sight, but the bullet got caught,” I observed.

“You think that snow would turn a bullet?”

I allowed that I thought so. Bullets are easily turned.
Compared to spears and arrows, bullets fly very fast. The first rifleman probably thought they also flew in a straight line. In 1537 Trataglia, an Italian scientist, described the bullet’s path as an arc. Trataglia determined that a launch angle of 45 degrees would give any bullet its greatest range. While his conclusion isn’t true for modern bullets, it was valid for low-velocity projectiles, which are affected more by gravity than by air resistance.

A century after Trataglia, Galileo dropped cannonballs from the Leaning Tower of Pisa and wrote that because gravity’s acceleration appeared constant, trajectories must be parabolic. He didn’t expound on drag (negligible on cannonballs dropped to earth). Another century would pass before anyone advanced the science of ballistics beyond Galileo’s work.

Meanwhile, Isaac Newton investigated laws of mechanics crucial to the understanding of ballistics. Newton’s universal law of gravitation held that the tug of gravity varies with altitude. He demonstrated that drag increases with the density of air and the cross-sectional area of a projectile. He also acknowledged the disproportionate effect of velocity on drag. Because he had no way to measure the speed of musket balls, he couldn’t know that drag increases dramatically when projectiles near the speed of sound (1,120 fps).

Around 1740, Englishman Benjamin Robins invented the ballistic pendulum. The pendulum had a heavy wooden bob of known weight, hung vertically. Robins weighed a bullet to be fired then shot it into the pendulum. By measuring the bob’s swing, he calculated impact velocity, because speed (with mass) is a component of momentum. People then could hardly believe musket balls flew as fast as his device showed: to 1,700 fps! Reduced readings at long range indicated air resistance to be 85 times as strong as the force of gravity! Incredible. But, of course, true.

We no longer measure velocity with pendulums, but with electric eyes that register the passage of a bullet’s shadow as the bullet travels a known distance. My friend Dr. Ken Oehler performed an enormous service for shooters by designing the first chronographs for consumer use.

The idea of a “standard bullet” that could be used to develop benchmark values for drag and other ballistic variables came to experimenters in the mid-19th century. The concept of ballistic coefficient followed. Commonly expressed as “C,” ballistic coefficient defines bullet flight in the equation C = drag deceleration of the standard bullet / drag deceleration of the actual bullet. Its mathematical expression employs sectional density, velocity and coefficient of form. Ballistic coefficients are useful in comparing flight characteristics of bullets only when those bullets are driven at the same speed. But few shooters have facilities for finding C in firing tests. A simple formula offers an alternative: C=w/id2, where “w” is the bullet weight in pounds, “d” is bullet diameter in inches and “I” is the form factor (a numerical expression of the bullet’s shape). The higher the ballistic coefficient, the flatter a bullet will fly and the better it will conserve speed and energy. The standard bullet has a C of 1.000. Most hunting bullets have Cs of .200 to .400.

Enough math. For a better understanding of bullet flight, you’ll crunch more numbers. But to shoot well at distance, you’ll also need a feel for outside conditions that affect flight. Here’s a short list.

For any given cartridge, bullet velocity depends on powder type and charge and bullet weight; it’s influenced by chamber and barrel dimensions, throat profile and length, and bore finish. A tight chamber reduces the energy lost to case expansion. So does a tight throat. A long throat permits shallow seating to increase powder space. But the farther a bullet moves before engaging the lands, the more likely a drop in pressure from gas that finds its way around the bullet.

Some barrels are “faster” than others, generally because their bore diameters are smaller. A clean barrel typically shoots to a different point of aim than one that’s fouled and often delivers higher or lower bullet speed than with subsequent rounds. Barrel length influences velocity because the bullet’s exit cuts short the pressure curve responsible for the bullet’s launch.

Components matter. Magnum primers, for instance, usually boost pressure. Similar change occurs with smaller powder chambers (thicker case walls or webs). Fast-burning powders abbreviate the pressure curve. Bullets with long shanks or hard or “sticky” jackets can elevate pressure. Expect velocity to follow pressure, though not at the same rate of increase.

The high exit speed of modern big-game rounds boosts the influence of drag on bullet flight. The sum of drag forces depends on a bullet’s weight and profile too, plus axial spin and atmospheric conditions. Add jacket texture at long range, because skin friction is a big component of drag. Pressure drag occurs at the bullet’s nose. A long ogive (conical section between bullet tip and shank) trims pressure drag—as does supersonic speed (greater than Mach 1.2). Subsonic travel (less than Mach 0.8) increases pressure drag. A supersonic bullet, however, also produces wave drag. Boat-tails or tapered bullet heels mitigate base drag, but you’ll notice that effect only beyond 300 yards or so.

Yaw sets up drag too. A bullet is in some state of yaw whenever its axis does not coincide exactly with its direction of travel. Precession, or the rotation of the bullet’s nose about its axis, adds to drag. Any tipping of the bullet—from a damaged muzzle, a nicked bullet base or lack of jacket concentricity—can put the nose into its own orbit around the bullet’s axis. Like a top that “goes to sleep” after you give it a spin, the bullet may rotate with less precession after covering some distance. That is why you may get smaller groups (in minutes of angle) at long range. Extreme yaw can cause the bullet to tumble. Axial spin fights yaw but must be matched to the bullet’s weight, length and speed. Rifling twist too
slow to stabilize the bullet can result in “keyholing” at the target. Excessive spin increases yaw angle.

Air resistance has a huge effect on bullet flight. Poke your head out the window of an automobile cruising at 60 mph, and you’ll identify with a bullet moving 88 feet per second. That’s glacial. Multiply the tug of that wind by 35, and you have the drag on a .300 Winchester Magnum bullet as it leaves the muzzle. That bullet must penetrate the atmosphere as it penetrates an elk. The air offers less resistance, but over a much longer period. The influence of other environmental factors—wind, humidity, temperature—are also  magnified by the bullet’s speed and time in
flight. Watch the BB from an air gun or the track of a rimfire bullet through a high-power scope, and you’ll see that neither path is straight! Aerial combat footage from World War II shows drift in the tracer bullets of machine guns.

As drag slows a bullet, gravity pulls it earthward. Every bullet falls at an accelerating rate of 32.16 feet per second after leaving the muzzle. One reason few bullets drop 32 feet is that they don’t stay aloft for a full second. Another is that we usually calculate drop not from bore-line but from sight-line, manipulated to intersect the bullet’s path at two points. The farthest is the zero range.

Unlike gravity, wind does not displace a bullet with accelerating force. Neither is it constant with regard to speed or direction. You can predict how far gravity will tug a bullet between 300 and 350 yards; but you cannot accurately predict displacement from wind because you cannot accurately gauge the wind over that segment of flight. Wind shifts at the muzzle have greater effect than do equivalent changes far off. But the bullet is more easily moved the farther it flies because there’s increasing time (per unit of distance traveled) for the wind to impose its will.

Wind direction, relative to the bullet path, matters as much as wind speed. “Full-value” wind, from 3 or 9 o’clock, has the greatest influence. Wind coming at you or from behind has the least effect. (A bullet clocking 3,000 fps generates its own headwind, a 2,000-mph gale—so a 20-mph breeze on nose or tail has negligible effect.) Wind from the right will give bullets from right-twist rifling a little lift as it moves point of impact to the left. Wind from the left depresses point of impact. You can shoot tight groups in wind—as long as it is steady. You can shoot accurately in wind—if you dope it accurately. It’s best to zero on a still day. That way, you can shade for wind effect afield. If you zero in wind, the lack of wind will displace your shot as surely as if it were blown off course. Beware the effect of topographic features on air movement. A mountain can bend the wind. Passes and canyons accelerate it.

How much drift? A useful rule of thumb is to assume an inch of drift for a 10-mph full-value wind at 100 yards, then double that at 200. Triple the 200 drift at 300, double the 300 drift at 400. Velocity and C value affect how closely a bullet will hew to this rule. Here’s how it applies to a 180-grain .30-06 bullet where AD is actual drift in inches and ROT is rule of thumb drift in inches.   

When doping wind, mind the mirage visible near the ground on warm days. Mirage boils straight up when there’s no wind, flattens during a pick-up. I found after many painful lessons in smallbore matches that boiling mirage can also signal a shift in wind, especially if it follows strong prevailing breeze. Shooting during a let-off, you run the risk of a pick-up just as you fire.

To shoot well in wind, you must practice in wind. Shoot at targets you can read (paper bulls-eyes). Match observations of wind speed (grasses bending, leaves ruffling, the press of air on your cheek) with bullet displacement. Consult a pocket anemometer like Kestrel’s to assign numbers. Complement practice with computer time using ballistics programs like Infinity from Sierra Bullets.

Such programs can also tell you how conditions you might not get during practice will affect your shots afield. Elevation, for example. The air in Wyoming elk country may be considerably thinner than at the rifle range outside Miami. Changes in trajectory at normal yardage are negligible, especially when your target is the size of an elk’s forward ribs. A 180-grain .30-06 bullet will strike only about 3 inches higher at 300 yards when fired at 10,000 feet after you zero the rifle at sea level. Air temperature also figures into the mix. Cold air is denser. So shooting at elk during November in the Rockies after prepping in Florida, you may automatically counter the effect of elevation. Figure no more than half a minute of elevation for a 100-degree change in temperature. Expect a velocity change of 3 fps for each degree F. Sierra’s software shows point of impact shift for a 140-grain Ballistic Tip bullet from a 7mm Remington Magnum: Started at 3,150 fps, that bullet fired in 59-degree air should land 16.9 inches low at 400 yards with a 200-yard zero. In 70-degree air, the bullet hits 16.8 inches low. You’d have to travel from the Carribean to the Arctic to notice any move in bullet impact caused by changes in air temperature, and then only at extreme range. Humidity likewise affects bullet arc to such a minor degree that you’ll be hard-pressed to notice it at normal ranges.

The temperature of the powder charge does matter. A hot day boosts temperature, which elevates higher pressure and velocity.  A cartridge kept in a warm pocket and fired soon after loading on a cold morning will behave differently than
one carried in a rifle all day in sub-zero temperature. Cartridges on a hot dashboard can get much warmer than the rifle, building higher pressures. Some powders are more temperature-sensitive than others. One selling point of the new Ruger Compact Magnum rounds developed by Hornady is their uniform behavior over a range of temperatures. Use your freezer to determine how cold will affect your ammunition.

Shot angle affects point of impact—noticeably at steep vertical angles and long range. Gravity has the greatest influence on bullet flight when the bullet is launched horizontally. Shooting straight down or straight up, you’re shooting into or against gravity, so it does not bend the bullet’s path. At angles between vertical and horizontal, you get something between zero and maximum effect from gravity. You can adjust appropriately by figuring the horizontal distance to your target, and holding for that. For example, if an elk is roughly 300 yards off, above you at a steep 45-degree angle, you’ll hold for 200 yards. That’s very close to the horizontal component of the yardage. A hold for 300 yards will cause the bullet to hit high. Inside a 200-yard zero, you can usually ignore shot angle.

At the last Elk Foundation Elk Camp in Reno, a fellow asked me how far he could shoot his .300 Magnum and kill an elk. I had to beg ignorance. “I’ve no idea how far you can shoot accurately.” Well, he pressed, how far is the .300 Winchester Magnum lethal? I shrugged. “As far as the remaining speed is sufficient to open an expanding bullet.” He walked away, apparently satisfied I knew nothing.

The lethal range of a cartridge has nothing to do with how far you can effectively kill elk, because conditions always undermine accuracy before the bullet slows to the point of terminal failure. I can say that with some certainty because, besides those conditions already covered here, there’s the human variable. It’s nonsense to focus on the effect of temperature or air density when the area scribed by a wandering reticle is bigger than the vitals of your target. From hunting positions, most riflemen are hard-pressed to shoot inside 3 minutes of angle consistently. Only gravity and wind move your bullet that much over normal distances.

Remember that a twitch of the rifle muzzle of only 1/50 inch displaces your bullet 6 inches at 200 yards. In the final sift, the condition to mind the most is the state of your marksmanship!