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Ballistic Coefficient Terms & Concepts – From Air Resistance & Boat Tail to G-Function

We’ve investigated the advantage of the BC (Ballistic Coefficient) in BC-Part 1, and we can see that velocity plays a key role in long range precision engagements. But what is BC and how is determined? First some terms and concepts.

Ballistic Coefficient Terms & Concepts

Air resistance is the frictional force that air exerts against a moving object. As an object such as a bullet moves, air resistance slows it down. The faster the object’s (in this case bullet’s) motion, the more air resistance is exerted against it.
Ballistic Coefficient (BC) is for bullets, what the drag coefficient is to aerodynamics in planes, cars and missiles. Like the drag coefficient, the BC is geometrically dependent, meaning shape and frontal area and the result is a dimensionless number. The ballistic coefficient compares the actual bullet to a standard bullet. Put succinctly the higher the BC the more slippery the bullet through the air. The formula is:
BC = SD/i where SD is the Sectional Density and i=the coefficient of form, or the form factor.
The BC is a primary factor is calculating bullet drop and other ballistic calculations. BC varies with velocity. Take the Sierra 150 grain Matchking, it lists a BC of .417 at 2700 fps and above. Between 1600 to 2800 fps it drops to BC of .397, below 1800 fps it is down to a BC of .355. Sierra Bullets is one of a very few sources that list multiple BCs for their bullets.
Boat-Tail refers to a tapered base versus the flat base of conventional bullets. Usually the taper is 7.5 degrees. As a bullet progresses through the atmosphere air flows along the body and spills over the trailing edge of the base, creating a turbulence causing a low-pressure area that retards the bullet. The boat tail bullet smooths the transition and lowers the base area subject to turbulence, increasing range and preserving velocity.
Caliber. The diameter of the bore, a rifled 30 caliber rifle requires a .308-inch bullet to file the grooves and provide a seal. Also use to define other measurement given in multiples of the bullets diameter to define the bullets length and curve of the ogive. In artillery, it also defines the barrel length as there is a correlation relating to range.
Drag. All objects moving through a fluid (liquids or gases) experience drag, retardation of movement due to friction and displacement of the fluid. The drag coefficient is used to calculate drag in most aerodynamic systems.
Drag Coefficient (Cd) = drag / density*velocity2*area (reference area) The Cd is the ratio of the drag force/dynamic pressure*area. Coefficients compare two quantities producing a dimensionless number, no units, just a number. It is geometrically dependent, i.e. geometric shape has a high effect on the drag coefficient. Sectional Density in ballistics defines much of same data for density and area.
Form Factor or Coefficient of Form (i), is the shape of the bullets ogive, or the curved portion forming the point. Long or short the front end of the bullet defines its form factor. Traditionally before electronic metering and computation was available the bullet was compared to a printed chart of logarithmic illustrations of bullet shape. Each shape was assigned a form factor number or coefficient of form value. Modern ballistic study incorporates doppler radar to determine the ballistic coefficient from which the coefficient of form can be derived.
G-Function: The G1 and G7 functions were derived by the French Gâve Commission. Using the Krupp standard projectile they developed the G1 function with through G8 functions plus a GL function to cover blunt cast lead projectiles following later. Some functions came latter but the comparison of a projectile to the standard projectile remains, just the specs of the standard projectile changes. G1 or Ingalls the most popular and is for a flat base with a fairly blunt ogive. The G7 is reserved for low drag long ogive bullets with a boat tail base.

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