If you look at Sierra Bullets website, you'll see that they list multiple B.C.'s for their bullets based on velocity ranges. That's because any projectile exhibits a different B.C. at different velocities let alone altitudes, barometric conditions, etc.
A single B.C. (usually highest number) is given based on the optimum velocity of the bullet at STP. Once outside of the sweet zone all bets are off. Also, for real long range shots, B.C. is only one of the factors that one needs to take into account. Another is the rounds inertia.
A 105 Howitzer...105x372R has a muzzle velocity of only 1550 fps. but it has a maximum range of 8300-8500 yards. We are talking almost 5 miles which is on the high side for a man portable weapon of much higher velocities. You may want to look up what a B.C. 1 round "IS". That was established quite a few years ago. As was referenced earlier...you'll want to look at the "G1" round.