Most experienced marksmen know that there is more to getting the bullet exactly where you want it to go than just what can be controlled with a good eye and a steady hand.
Although these factors are certainly critical, Mother Nature has introduced a number of other elements into the ballistic equation that are mostly beyond the shooter’s control. This doesn’t mean that the strike point of the bullet is a largely random event, but rather that a successful shooter has to take these dynamics into consideration.
These physical factors don’t have much influence on practical handgun shooting. If the target is seven yards away, aerodynamics, wind, and atmospherics aren’t going to matter much. The bullet is in flight for such a short time, the point of aim is going to be very close to the bullet’s strike point. They matter more with rifle distances of 100 yards and more, and they’re critical with really long-range targets.
1. Aerodynamics
“Bullet flight” is a misnomer, because bullets don’t fly any more than basketballs or footballs do. To fly, the object in flight has to have some kind of airfoil that causes air to flow around it in a way so that lift is generated.
Bullets have a uniform cross-section, and they rotate in ‘flight’ so even if there was an airfoil, it would be in the wrong position to generate lift most of the time. When a bullet leaves the gun barrel, it immediately becomes subject to the force of gravity, pulling it downward.
Imagine you could find a range long enough and had some way to measure when your fired bullet hit the ground. You would find that a bullet dropped from the same height as the gun barrel would hit the ground at the same time a fired bullet did. The flight path would look a lot like a missed football pass — arcing toward the ground.
This doesn’t mean that aerodynamics is irrelevant to the flight of the bullet. In fact, the shape of the bullet has considerable impact on its course of flight because of aerodynamic drag. The bullet is going to meet resistance from the air as it moves along its course of flight. This causes the bullet to begin to decelerate immediately upon leaving the gun barrel, where there are gases that propel and accelerate it.
The moment that the pressure of those gases is not contained by the barrel, the pressure of the air in front of the bullet will begin to reduce its velocity, gradually slowing it down. This has some effect on the terminal effectiveness of the bulletpenetrating the target, but the reduction in velocity will increase the time it takes the bullet to get to the target.. While the bullet is decelerating horizontally, it’s accelerating vertically, as gravity draws it closer to terra firma.
a. Ballistic coefficient - The aerodynamic drag that a bullet will experience is determined largely by its ballistic coefficient, a number between zero and one. Smaller, sleeker bullets have a high ballistic factor, where larger, heavier bullets have a lower one. The lower the ballistic coefficient, the greater the aerodynamic drag on the bullet and the more the shooter will have to compensate over long ranges.
Most manufacturers will supply the ballistic coefficient of their products on request, and this number is extremely useful in calculating the optimum aiming point for the rifleman.
2. Wind and gyroscopic stability
The most dramatic effect of wind on the flight of a bullet is to change its direction horizontally. Wind will have the greatest effect when its direction is at a right angle to the path of the bullet, and the wind speed is relatively high. As the wind direction moves to the same plane as the bullet flight (wind from either the twelve o’clock or the six o’clock direction, relative to the shooter), its effect becomes more negligible.
The rounded shape of the bullet works to its advantage in combating wind drift, but the largest cross-section of the bullet is still the one that is most exposed to a crosswind, and there is some effect on the bullet path.
Calculating the impact of this effect is complicated. The direction and speed of the wind is seldom uniform over the entire flight of the bullet. In fact, it would be — at best — difficult to know the speed and direction of wind along the entire flight path of the bullet, so the shooter usually has to settle for getting an idea of the wind speed and direction at the firing line.
Image there is a 10-mile-per-hour crosswind, blowing from three o’clock to nine o’clock at the firing line. The cross-component of this wind is 10 MPH, because the direction is at a right angle to the bullet flight path.
If the wind is from two o’clock to eight o’clock, the cross-component would be reduced to around 8.7 MPH — or the same as an 8.7 mph wind blowing from three o’clock to nine o’clock. The wind at the firing line is going to move the bullet slightly to the left — in this example — and curve its path slightly to the left as well.
a. Gyroscopic inertia and Precession - The effect of wind is greatly reduced by the spinning action of the bullet that is imparted by the grooves in the gun barrel as the bullet is expelled. This is called gyroscopic inertia, and is the same principle that governs a bicycle or motorcycle in motion. A bicycle wheel will fall over if placed on its rim while motionless, but will remain on the rim once the wheel begins spinning. The faster the wheel spins, the greater its resistance to forces outside of the spin plane. If the bicycle rider wishes to turn, it is easier to do so by leaning in the direction of the turn than it is to try to turn the wheel itself. This is because the inertial forces are greatest at the center of the wheel and weakest at its perimeter. Changing the direction of the wheel in this manner is called precession.
If not for some precession, a bullet would move along its intended path until it ran out of momentum completely, and then drop to the ground.
Precession also contributes to some variance from the straight-line path because the bullet is pointed in a direction slightly off from its flight path. A bullet fired with the barrel parallel to the ground will be pointed in the horizontal direction of its flight path, but will actually be traveling along this downward arc.
b. Yaw The difference between the angle of the bullet’s axis from nose to tail and the axis of the flight path is called yaw. The more or less steady attitude that the bullet assumes shortly after firing — the firing itself and disturbances as the bullet leaves the gun barrel contribute to some variances here before the bullet settles into a steady attitude — is called the yaw of repose.
At the yaw of repose, the bullet will be pointed slightly upward and to the right if fired from a barrel with a right hand twist, and slightly upward and to the left if fired from a barrel with a left hand twist. This horizontal drift is created because the spinning action of the bullet makes for an increase of air pressure on the left side of the bullet as compared to the right, pushing the bullet slightly to the right.
3. Air Temperature, Density, and Humidity
Temperature affects projectile performance because warm air is less dense than cool air, and sound moves more slowly in cold air than in warm air. Most substances contract when cooled, and air is no exception, becoming denser. This is illustrated by measuring the speed of sound in warm air versus cold air, and finding that more kinetic energy is required to break the sound barrier in cold air. Most rifle — and some handgun — bullets are supersonic, and are thus subject to this phenomenon.
Air density is measured with a barometer, and is primarily dependent on altitude above or below sea level. There are also variations in air density caused by “high pressure” or “low pressure” fronts that accompany changes in weather, and a drop in barometric pressure is usually an indicator of an approaching storm.
If a shooter wishes to include the effect of air density in his or her calculations for optimum aiming point, they need to use a barometer calibrated to actual barometric pressure as opposed to relative barometric pressure.
Actual barometric pressure will be reduced by about one inch of mercury — sometimes abbreviated on the barometer as “in Hg” or “inches Hg” or “Hg” being the chemical symbol for mercury — for every 1,000 feet of increase in altitude.
Finally, relative humidity changes the performance of a bullet in flight because moist or humid air is less dense than dry air. This runs in the face of logic, because we perceive humid air as being thick and dense as compared to dry air. However, the molecular weight of water (hydrogen and oxygen – hydrogen is the lightest element) is less than the molecular weight of nitrogen and oxygen, the gases that make up the bulk of our breathable air.
The effect of relative humidity is greater at high temperatures than at low temperatures, but is not substantial in either case. The difference in density of completely dry air (zero percent humidity) and completely saturated air (100 percent humidity) at 90° F is only about one percent.
