External Ballistics

 

External Ballistics is the science of the bullet in flight. It involves EVERYTHING that occurs from the instant the bullet leaves the muzzle until it impacts the target. An understanding of those factors and effects of external ballistics is imperative. Sound marksmanship, experience, and a solid understanding of those factors and effects will enable the Sniper to compensate and hit the intended target.
Two forces influence the bullet in flight. One is gravity, which is constant. The other is air resistance, more commonly called "drag". Of the two, gravity has the least effect. If the .30 caliber bullet was fired in a gravity free atmosphere, it would travel slightly less than two miles and stop in mid air. If the same bullet was fired in a vacuum, but with gravity present, it would travel about 43 miles and strike the earth at the same velocity it left the muzzle. In a world of REALITY where both forces exist, gravity pulls the bullet down while drag simultaneously slows the bullet.
Gravity is a constant acceleration of the bullet downward at the rate of 32 feet per second, per second. A simplified example of this is after one second the bullet falls at a velocity of 32 FPS, after two seconds at a velocity of 64 FPS, etc. IT ACTS INDEPENDENTLY of the bullet's weight, shape, or velocity from the muzzle. The instant a bullet exits the muzzle, gravity accelerates it down at the constant rate of 32 FPS, per second. Theoretically, if a bullet was dropped from the beside the muzzle at the exact same instant an identical bullet was fired from the muzzle, they would both hit the ground at the same time, albeit widely spread apart. This is true in a vacuum. IN REAL LIFE, the fired bullet lands after the dropped bullet because it generates some lift as it flies through the air. The lift counteracts gravity to a small degree.
The longer the time of flight, the faster the bullet's falling velocity becomes until it reaches a terminal velocity on the order of 250 FPS. "Terminal Velocity" is that velocity at which drag has increased to the point gravitational acceleration is counteracted. Bullets do not have a time of flight long enough to reach terminal velocity as they drop. A typical .30 caliber bullet takes approximately 1/10 of a second to travel 200 yards.

Drag works in opposition to the direction of velocity. Although all objects fall at the same rate regardless of shape, size, or weight, they do not all reach the same terminal velocity because drag varies with shape, weight, and surface area. Thus a feather and a rock do not hit the ground at the same time when dropped simultaneously although they would if dropped in a vacuum.
Gravity gives a bullet's line of flight, called the trajectory, its curving shape. The trajectory of a bullet is a parabolic curve, which is defined as a constantly increasing curve. The slope of the curve becomes steeper as the range increases. The distance a bullet is below the line of bore is called drop. Since the trajectory is a parabolic curve, the drop increases with range. At the muzzle the drop is zero. For example, a flat base .308 bullet at 100 yards, the drop is approximately 2", at 200, approximately 11 inches, and at 1000 yards, approximately 480 inches.
In order to hit a target, the line of bore, as represented by the barrel, must be angled up relative to the horizontal. This angle is called the "angle of departure", typically 3 degrees for a 308 at 1000 yards. The result is that the bullet will cross the shooters line of sight twice.
Line of sight is a straight line through the sights to the target and is above the barrel typically 1.5" above the muzzle. With this, the bullet typically crosses the line of sight around 22-25 yards.
The second intersection of the line of sight is the zero point of the rifle. The zero point is altered by raising or lowering one of the sights until it coincides with the desired zero for the weapon. The actual dimensions are very small, 1 click is 1/4 moa, yet the effects can be monumental. For the 3 degree angle at 1000 yards, any small error can have a large effect.
Velocity is the only factor that influences the perceived effects of gravity and then ONLY relative to a specific range. Gravity acts on the bullet ONLY for the duration of its HORIZONTAL flight vector. The faster a bullet travels, the less time gravity has to effect it. If a bullet takes 1 second to reach its target it will drop 32 feet. A faster bullet reaching the same target in 1/2 second will only drop 16 feet.

The second force which influences the bullets flight is drag. Drag resists velocity, increasing exponentially as velocity increases. The higher the velocity, the greater the rate at which velocity is lost.
An example, a 180 grain 308 flat base bullet with a muzzle velocity of 2100 FPS will have a retained velocity at 1000 yards of 1045 FPS, a loss of about 50%. The same bullet with a muzzle velocity of 3200 FPS will have a retained velocity of 1433fps, a loss of 55%. The faster bullet has a higher rate of velocity loss, but it has a greater retained velocity because it had a greater intitial velocity. This illustrates the increase of DRAG with an increase of velocity. This effects trajectory since the time of flight decays, thereby giving gravity more time to act on the bullet.
DRAG represents ENERGY given up by the bullet pushing the air aside as it flies through the air. The air is compressed by the nose of the bullet and forced out of the way. As the air stream moves down the sides of the bullet, friction with the sides causes TURBULENCE. The turbulence drags against the bullet, another component of the retarding effect of drag. As the air spills off the rear of the bullet, more turbulence is created along with a partial vacuum at the base, both of which further slow the bullet. Bullet shape and design are the important factors relative to drag.
Bullets are designed for various purposes and a aerodynamically efficient design , or sharp pointed bullet is far more efficient relative to drag.
The aerodynamic efficiency of a bullet is expressed as BALLISTIC COEFFICIENT, a three digit number, less than the number one, which denotes the bullets ability to overcome drag. The higher the number, the more efficient, and although impossible, a bullet reaching the number 1 would not lose any velocity to drag.
Ballistic coefficient is independent of bullet weight and caliber in its effect, although increasing weight in a specific caliber will increase ballistic coefficient.
Aerodynamically, a boattail bullet is more efficient at longer ranges, with less drag in their flight. The air stream spills off the tapered rear of the bullet more smoothly, creating less turbulence and less vacuum. A small hollow point will reduce the compaction of the air in front of the bullet and result in less turbulence, and act like a small cookie cutter against the denser compacted air, thus reducing drag. Reduced drag results in a flatter trajectory.

Once trajectory is understood, the rifle can be fired accurately. Knowing trajectory can allow a shooter to know where to aim or what sight adjustment to make for distant shots. The match standard, a .308 168 grain hollow point Sierra Match bullet at 2650 FPS with a BC of .475, is graded against a standard temperature/barometric (air) pressure. The standard temperature is 59 degrees F at sea level and 29.92 inches of mercury. Velocity and trajectory will vary with different temperatures and air pressures. With this information, ballistic tables can be created with a great degree of accuracy.
Knowing the trajectory is only half the problem, the distance must also be known to hit your target.
Although Gravity and Drag are the only forces which act on the TRAJECTORY, there ARE EXTERNAL factors which INFLUENCE trajectory "relative to the point of aim".

These factors are WIND, ALTITUDE, TEMPERATURE, HUMIDITY, and BAROMETRIC PRESSURE. OF these WIND is by far the MOST SIGNIFICANT.

WIND IS NOTHING MORE THAN AIR IN MOTION. Since the bullet is moving in the air, as the air moves, so does the bullet. When the wind is blowing at a right angle to the bullet's line of flight, the HORIZONTAL deflection will be the greatest. Wind deflection is ALWAYS in the SAME DIRECTION as the wind. Deflection DECREASES as the angle of the wind to the line of bullet flight to decreases, less angle, less wind, less deflection. When the wind is parallel to the line of flight (either a head wind or a tailwind) HORIZONTAL DEFLECTION is ZERO. Some VERTICAL DEFLECTION will occur with either a head wind or tailwind, but it is noticeable primarily at ranges exceeding 600 yards. This VERTICAL DEFLECTION is caused by the bullet velocity being augmented by the tailwind velocity or diminished by the head wind velocity.

The AMOUNT of DEFLECTION caused by the wind is determined by the direction of the wind, its velocity, and the range to the target. The greater the range, the longer the wind will have to move the bullet. And the faster the wind blows, the faster it will move the bullet. Wind deflection is NOT a constant curve. Like trajectory, wind deflection is a Parabolic Curve, constantly increasing. With the mentioned 168 bullet, a 1 mph wind change can move the bullet 7".
Although Wind deflection varies with the angle of the wind to the line of bullet flight, the difference between related angles is small. Since the greatest deflection occurs at right angle winds, the deflection must be learned. Winds are classified according to the direction from which they are blowing. Right angle winds are full value winds, which means the full value of the deflection will apply. Winds at less than right angles are half value winds and less than full deflection is applied. Head winds and tailwinds cause no significant deflection like right angle winds do and are called zero value winds, however, they may cause vertical deflection as mentioned earlier.
To shoot accurately in the wind, the Sniper must know wind velocity, wind direction, and the full value deflection for the range he is shooting. If the wind is full value, a full value correction must be made, a half value wind, a half value correction, whether by hold offs of by a sight correction.
The Sniper must have a way to measure Wind Velocity. Wind tables are available to make deflection corrections.
Because of obstructions and terrain features such as buildings, tree lines, hill or alleys, the wind can be blowing from several different directions between the Sniper and the target. The differences CAN be EXTREME. At the Snipers position, there could be a full value wind from the left, while a half value wind from the right could be blowing at the target. Since wind deflection is minimal close to the Sniper and maximal at the target, it is necessary to determine the wind direction and velocity at a point in between which will best average out the effect. This called READING THE WIND.
As a rule, read the wind at a point between 2/3 and 3/4 of the way to the target. Estimate the direction and velocity at that point and apply corrections accordingly. Although tables available to the Sniper give wind deflection values, as do the ballistic tables and programs which give wind deflections for a range of velocities and directions, THERE IS NO SUBSTITUTE FOR PRACTICE AND EXPERIENCE. The table, charts, and programs are APPROXIMATIONS AT BEST. IN ORDER to LEARN how to read wind and SHOOT WELL in wind, the Sniper must PRACTICE in windy conditions and KEEP ACCURATE RECORDS of the EFFECTS on him and his shooting. (FBI SOURCE)

This is the best written explanation that can be find from the US ARMY, the USMC, and the FBI on External Ballistics and the effects of winds. It has been the training doctrine of the listed entities for their Sniper Program for a number of successful years. I hope some of you find it helpful.

 

Articles