The drag coefficient cD is the most important aerodynamic coefficient and generally depends on
 bullet geometry (symbolic variable B),The following assumptions and simplifications are usually made in ballistics:
a) c_{D}(B,Ma,d) = c_{Do}(B,Ma) + c_{Dd}(B,Ma) * d^{2}/2
Another theory which accounts for arbitrary angles of yaw is called the "crossflow analogy prediction method". A discussion of this method is far beyond the scope of this article, however the general type of equation for the drag coefficient is as follows:
b) c_{D}(B,Ma,d) = c_{Do}(B,Ma) + F(B,Ma,Re,d)
Fig.: Zeroyaw drag coefficient for two military bullets
M80 (cal. 7.62 x 51 Nato)
SS109 (cal. 5.56 x 45)
There is also software available which estimates the zeroyaw drag coefficient
as a function of the Mach number from bullet geometry. The latter method
is mainly applied in the development phase of a new projectile.
c_{Do}(B,Ma) = i_{D}(B) * c_{Do }^{standard}(Ma)
If this simplification is applicable, the determination of the drag coefficient of a bullet as a function of the Mach number is reduced to the determination of a suitable form factor alone. It will be shown that the concept of the ballistic coefficient, widely used in the US for small arms projectiles follows this idea.

Drag coefficient; c_{D}(B,Ma,Re,d) 

Zeroyaw standard drag function 

Form factor 