Campfire bwana,
A BC is a measure of how well a bullet coasts through air. The higher the BC, the less deceleration due to drag force it experiences at any given Mach number in any given air density. For any pair of bullets with the same diameter and profile, drag due to shape is the same at all matching velocities in the same atmospheric conditions. However, if one bullet with that profile is made from a material twice as dense as the other, it will have twice as much mass, and as a result, that matching drag force will only be able to slow the double-mass bullet half as quickly. This means the double-mass bullet coasts twice as far between any two given velocity values as the single-mass bullet does, and thus has twice the ballistic coefficient of the single-mass bullet. It also means the double-mass bullet has twice the sectional density of the singly-dense bullet. This is how sectional density relates to BC.
Note that nowhere in that discussion did I have to specify a particular velocity to find the relative BCs because we knew the shape-dependent drag would match at all velocities in a given set of conditions. Indeed, the reference projectile system of BCs developed by the Gavre Commission in the 19th century and later modified with numerous reference projectile shapes by the US Army Ballistics Research Laboratory takes advantage of shape similarities even when they are scaled to different diameters. All the BRL shapes are modeled to be one inch in diameter and to have a weight of one pound. This means they all have a ballistic sectional density of one. Thus, when you shoot a smaller bullet of the same shape, the difference in the rate at which it slows between two given velocities as compared to how quickly the reference projectile would slow is proportional to its sectional density divided by the reference projectile's sectional density. Since the reference projectile's sectional density is one, and any number divided by one is itself, you can skip the division and the smaller projectile's BC for that reference projectile's drag curve is simply equal to its sectional density.
Thus far, all I have discussed are matching bullet shapes. When the shapes don't match, the small bullet's sectional density has to be divided by a shape difference coefficient called the form factor to account for the mismatch in shape-dependent drag. Since different shapes don't have the same shape drag vs Mach number curves, in order that the BC of a bullet relative to a reference projectile of mismatched shape does not wander too far off the reference drag curve, the form factor has to be adjusted for different velocity ranges, and thus the BC changes at different velocities. This results in the tables you see of multiple BCs for different velocities spans. This may be what you meant about having to have a bullet in motion to define a BC.
If you look in the Wikipedia entry for ballistic coefficient, you will find the math that matches what I have described, but you do have to stick to the ballistic definition of a BC and not wander off into the aerodynamics definition.
