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
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
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.