Quote:
And notice how it all varies as a function of speed (Mach number).
A single B.C. is valid over only a small section of the velocity envelope.
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Not exactly, although it is kind of true.
You are confusing the coefficient of drag (Cd), which is what the graph is of, and BC. Cd absolutely changes as a function of mach. BC only changes as a function of mach when the profile of the bullet diverges significantly from the profile of the base Cd curve, e.g. the G1 projectile.
If we use the G1 BC as example: BC is the ratio of "this" bullet's Cd curve to the G1 bullet's Cd curve. Yes, the ratio of "this" Cd to the G1 Cd at any place on the mach axis may not be the same. If you graphed the ratio of the two Cd curves (ie the BC), it might fluctuate a small amount around the "BC" value. However, for projectiles that follow more or less the G1 profile, it will be more or less the same throughout the mach range.
If anyone wants to dive into this, I recommend the McCoy book. It is "the bible" on external ballistics of symmetric projectiles.