External Ballistics


Chicks Dig The Long Shot
Verified Sniper
Jan 4, 2008
New Jersey
We have the terminal ballistics discussion so I figured I'd kick off the other two with a cut and paste from my physics presentation. It's basic and was geared toward "hunters" to keep the sheep happy.

Please correct, add, or delete anything wrong, unclear, or TOO informative as per OPSEC concerns.

Science is very important in accurate shooting. Common misconception is that the precise placement of a round is a result of mysterious forces, super human talent, or even pure luck. Some even think that the extreme accuracy in sniping is impossible. In truth, consistent application of marksmanship and proper application of science, would allow a hunter to bring down even a running deer with confidence at 300 yards.

Many hunters rarely understand all that is involved when firing their rifles, despite their extreme confidence in themselves. A well educated and efficient hunter should understand all the ballistics and adjustments that goes into every shot.

Ballistics are divided into three distinct types: internal, external, and terminal.

External ballistics is the flight of the bullet in the air. It is what happens from the time the bullet leaves the muzzle until it impacts something that is, hopefully, the intended target. Two main factors that influence the bullet in flight is gravity, which we know is constant, and air resistance, which we call drag.

Gravity pulls the bullet down at a constant rate of 32 feet per second and acts independently of the bullet’s weight, shape, or velocity. The instant it leaves the muzzle gravity accelerates it down until it reaches a terminal velocity of 250 feet per second. Terminal velocity is that velocity at which drag has increased to the point of counteracting gravitational acceleration.

Gravity is what gives the bullet’s line of flight 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 range increases. The distance a bullet is below the line of bore is called “drop”. The drop also increases with range. In order to hit a target, this line of bore, as represented by the barrel, must be angled up. This is called the “angle of departure.” The result is that the bullet crosses the shooter’s line of sight twice.

Line of sight is a straight line through the sights to the target, and is above the barrel. The bullet crosses it once close to the muzzle on the way up, and again at some point down grange on the way down. With scoped rifles, the line of sight is typically 1.5 inches above the muzzle and the bullet first crosses it at a range between 22 and 25 yards. The second intersection is usually 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. This changes the angle of departure and the arc of the round causing the point at which the bullet crosses the line of sight accordingly.

These changes can be accurately made by a variety of systems of measurement, the most widely accepted being minutes of angle, or MOA. The 360 degrees of a circle are broken down in increments called minutes. A degree has 60 minutes, which equates to 21600 minutes in a circle. This is illustrated in shooting as: from the muzzle to a point 100 yards away, one minute of angle will equal 1 inch. Like a piece of pie, the farther you get from the muzzle, the wider the area covered by one minute of angle. At 200 yards, one minute of angle covers 2 inches. At 300 yards, 3 inches. Thus, one MOA equals 1 inch per 100 yards of distance. This is an approximation because one MOA actually 1.047 inches at 100 yards.

Typical rifle scopes have adjustments, or “clicks” that equal some measure of MOA, usually ¼ or ½. If you were to make a 3 click adjustment with a ½ MOA scope, at 100 yards, the strike of the round would shift 1.5 inches, provided your rifle, ammo, scope, and shooter are all able to maintain that level of accuracy. In order for a weapon to be classified as a sniper rifle, it must be capable of shooting 1 MOA. That is every single round able to group together inside a 1 inch by 1 inch square from across a football field. The SVD rifle, commonly known as a “Dragunov” is actually not a true sniper rifle because it is only capable of 2 MOA in an exceptional shooters hands.

Drag works in opposition to the direction of velocity. It varies accordingly to the shape, weight, and surface area of the bullet. Drag resists velocity, increasing exponentially as velocity increases. The higher the velocity, the greater the drag and the greater the rate at which velocity is lost. Simply put, drag can be what effects the atmosphere is putting on the bullet. Wind, temperature, barometric pressure, altitude, and humidity are some examples.

By a simple law of nature which we call Newton’s First Law of Motion, any cross-velocity imparted early is retained throughout the rest of the flight time. The cross-velocity imparted at or near the target simply has little or no time to accumulate a displacement. Thus, the wind near the shooter will have a greater effect on the round as opposed to the wind near the target. With careful calculation of wind speed and direction, you can determine the exact adjustment needed to place the first round on target. A head or tail wind in relation to the shooter will have zero effect on the bullet. It is deflected greatest by a right angle wind, called “full-value wind”. Winds only 60 degrees from any right angle are termed “half-value wind” and will only deflected 50% of the distance were the same wind of full value.

Temperature can have considerable effect on the muzzle velocity and trajectory of a bullet, especially in a climate like the one we live in. Ammunition will assume the ambient temperature of the environment. The hotter it is, the higher the pressures created by the combustion of the powder and the faster the resultant muzzle velocity. A faster muzzle velocity will push the round farther, causing it to strike higher on target. A 30 degree change in ambient temperature will change muzzle velocity by 50 feet per second and a 20 degree change in temperature will change the strike of the round by one minute. The temperature correlates to barometric pressure, which is the measure of air density. A bullet will cut through colder, denser air with greater difficulty, resulting in a lower striking round as well as lower velocity imparted on contact.

For example, if you were to zero your minute of angle hunting rifle chambered for .308 Winchester at 200 yards on a comfortable 59 degrees fall day, on impact the retained velocity of the bullet would be 2,255 feet per second. However, you might find yourself at the end of the season sighting in on your trophy buck at dawn and the temperature is a crisp 25 degrees. If you were to fire at the deer that is 200 yards away, the approximate retained velocity would be 2,100 feet per second and the round will strike about 4.5 inches low. With a caliber like the .308 and depending on the terminal ballistics, that is enough difference for the deer to survive long enough to run far enough to not be found.
Oh, I should just throw out there that I've never actually hunted any animals, so forgive me if my deer assumptions are innaccurate.