Quote Originally Posted by C Ward View Post
Bullet weight has nothing to do with the BC of the bullet . The BC is a function of length and shape , the weight is a byproduct of this because longer bullets are typically heavier .

The Sierra 155 and 175 Match Kings and the 155 Scenar all have effectively the same BC number of .495 .

BC is what matters because that is the model for how well the bullet fly's.
I didn't want to get technical but I'll clarify. Look at the equation for BC, which is directly proportional to Mass. It can be rewritten as a function of the density (p), length (l) and drag coefficient (Cd). The equation shows that mass increase is proportional to BC increase, or as you said when the bullet length increases.



Alternatively BC is written for bullets, and again, is directly proportional to mass:



To get to my point, when you transition from supersonic to subsonic flight the aerodynamic characteristics change. These characteristics are described by the drag coefficient (Cd). The equation for Cd is shown below. The density (p) of the fluid (air) is higher during supersonic flight due to compressive shockwaves around the bullet. Note also that the bullet drag changes with the square of the bullet's velocity relative to the fluid velocity.



There is inherent transonic instability around Mach 1 and a resulting transition in these parameters. The ballistic coefficient is constant in a simple model, and doesn't take into account the many nonlinear effects at the transition from supersonic to subsonic flight. Hence when a bullet transitions to subsonic flight the air density and relative velocity see a significant change. This influences BC, which influences the point of impact. Therefore, it is desirable to maintain supersonic flight through a bullet's entire trajectory, as is typically accomplished with a heavier bullet.