The ballistic coefficient of a bullet can vary over .060 when shot out of the same gun with a variety of different loads. One could imagine a large variance for a particular bullet when shot out of different cartridges and different guns.
When hunting in Wyoming at 5000 feet above sea level the 30 Wolf will shoot the 178 gr. Amax 18 MOA up from 100 yards to 1000 yards. At sea level it is 20 minutes up from 100 yards to 1000 yards.
A bullet with a properly designed boat tail, and a favorable relationship of center of pressure to center of gravity will actually see a rise in ballistic coefficient at long-range. A bullet that is not designed optimally will usually see the ballistic coefficient decrease as velocity decreases.
one must shoot a particular bullet out of their own gun to determine what the actual ballistic coefficient will be.