The ballistic coefficient refers to how well a bullet maintains velocity [how streamline it is which is affected by shape and to some extent sectional density]. Or in other words how Aerodynamic the bullet is.
Sectional Density is about how dense a bullet is in the cross section and other factors being equal [like bullet construction and impact velocity] bullets of equal SD will penetrate the same amount.
Sectional Density [SD]= (bullet mass in grains/7000) / [bore diameter in inches squared]
SD example for a 180 grain .308" bullet
180/7000=0.0257
.308 * .308=.0948
SD= (180/7000)/(.308 * .308)
SD= .0257/.0948
SD for 180 grain .308" bullet=0.271
Hope that was clear?
Now the Ballistic Coefficient of a 180 grain .308" bullet can be different things depending on shape and what velocity range you measure it over [drag varies with speed in air anyway IIRC] usually BC are estimated with an equation. But for some Speer 180 grain .308" bullets they [Speer] list BC's of "0.304", "0.540", "0.483", "0.352", "0.416" in their Number 12 reloading manual.
Did that help any? hard to explain it over the net. If you have questions email me and I will give you links to Sierra or such I think they explain it there. Or pick up a reloading manual they cover most of this stuff in great depth.
