http://www.jbmballistics.com/ballistics/topics/secdens.shtml Sectional Density and Ballistic Coefficients
Sectional density equations and calculations. References and a list of variables are at the bottom of this page.
The BC, or ballistic coefficient is defined as:
BC = w / [i d2]
here the diameter is specified in inches and the weight in pounds and the form factor is found using:
i = CD / CDG
The sectional density is defined as:
SD = w / d2
making the ballistic coefficient
BC = SD / i
[NOTE: Some references define the sectional density with the mass not weight. All the listings I've seen from bullet manufacturers use weight. Most ballistics texts use mass.]
So this means that the ballistic coefficient is proportional to the weight of the bullet and inversely proportional to the diameter squared. (Keep in mind that the ballistic coefficient is also inversely proportional to the form factor which depends on the shape of the bullet!)
Calculation of the sectional density is straight forward. For a 300 grain, .338 caliber bullet, the sectional density is:
SD = [ 300 gr / (7000 gr/lb) ] / [ 0.338 in ]2 = 0.375 lb/in2
NOTE: With the common definition of the sectional density, the units have to be converted when used with drag functions, velocity, etc, to convert the in2 to ft2 resulting in a factor of 144.
Variables
d bullet diameter w bullet weight
SD sectional density BC ballistic coefficient
i form factor G "G" function
CD drag coefficient CDG drag coefficient of the standard bullet
References
Hatcher's Notebook, Julian S. Hatcher, Major General, USA, retired, Stackpole Books, Harrisburg, Pennsylvania, Second printing, 1966.
