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SO WHAT REALLY IS A B.C.?
The Coefficient of Drag (C.D.) for a bullet is an aerodynamic factor that relates air drag to air density, cross-sectional area, velocity and mass. One way to view C.D. is as the "generic indicator" of drag for any bullet of the same shape. Sectional Density (weight multiplied by it's frontal area) can then be used to relate the drag coefficient to different bullet sizes.
Sectional Density = (Wt. in Grains/7,000) / (Dia.* Dia.)
You can see from the formula that a 1 inch diameter, 1 pound bullet (7,000 gr.) would produce a sectional density of 1. Indeed the standard projectile for all drag functions always weighs 1 pound with a 1 inch diameter.
Another term occasionally found in load manuals is the bullet's "Form Factor". The form factor is simply the C.D. of a bullet divided by the C.D. of a pre-defined drag function's standard reference projectile.
Form Factor = (C.D. of any bullet) / (C.D. of the Defined 'G' Function Std. Bullet)
Ballistic Coefficients are then the ratio of velocity retardation due to air drag (or C.D.) for a particular bullet to that of its larger 'G' Model standard reference projectile. To relate the size of the bullet to that of the standard projectile we simply divide the bullet's sectional density by it's form factor.
Ballistic Coefficient = (Bullet Sectional Density) / (Bullet Form Factor)
From these short formulae it is evident that a bullet with the same shape as any standard bullet, weighing 1 lb. and 1 inch in diameter will always have a B.C. of 1.000. If the bullet is the same shape, but smaller, it will have an identical C.D., with a form factor of 1.000 and a B.C. equal to it's sectional density. You often see articles in magazines declaring the B.C. is simply the Sectional Density divided by Form Factor. They rarely explain a form factor requires accurate C.D. measurements for the bullet and also a standard reference projectile OF THE SAME SHAPE. Indeed if it were that easy to calculate B.C.'s all software would have the ability to calculate B.C.'s for bullets, but few products can..
Here are some drag models commonly used in small arms ballistics:
G1.1 - Standard model, Flat Based with 2 caliber (blunt) nose ogive
G5.1 - For Moderate (low base) Boat Tails - 7° 30' Tail Taper with 6.19 caliber tangent nose ogive
G6.1 - For flat based "Spire Point" type bullets - 6.09 caliber secant nose ogive
G7.1 - For "VLD" type Boat Tails - long 7° 30' Tail Taper with 10 caliber tangent nose ogive
GS - For round ball - Based on measured 9/16" spherical projectiles
RA4 - For 22 Long Rifle, identical to G1 below 1400 fps
GL - Traditional model used for blunt nosed exposed lead bullets, identical to G1 below 1400 fps
GI - Converted from the original Ingalls tables
FOR BEST ACCURACY, CALCULATE YOUR OWN COEFFICIENTS!
Accurate B.C.'s are crucial to getting good data from your exterior ballistics software. A good ballistic program should be able to use two velocities and the distance between them to calculate an exact ballistic coefficient for any of the common drag models. This method of calculating a B.C. is preferred and can be used to duplicate published velocity tables for a bullet when the coefficient is unknown or to more accurately model trajectories from your own firearm. A lot has changed in exterior ballistics since the 1870's. If your software does not allow you to select various drag functions, chances are it does not employ the latest modeling techniques and cannot calculate B.C.'s or generate an accurate long range trajectory for all bullets.
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