Manufacturers do not use the same methods to determine the ballistic coefficient of their bullets.  Any comparison with the BC values of other manufacturers is not valid as there is currently no standard method in use between manufacturers.  In fact, there are many different methods in use by those who say that the methods used by the manufacturers are not uniform.

BC changes as the speed of the bullet changes.  The terminology used is supersonic, transsonic (around the speed of sound, depending on elevation) and subsonic (lower than the speed of sound).

There are three BC values on the GSC website and they are only for use in an external ballistics program, not for comparison. Use the three values, as given, to approximately calculate BC for your launch speed and distance. Use a ballistics program that asks for more than one G1 BC.  A program that asks for one G1 BC figure will just give an approximate result.  The common fallacy is that one can convert from one type of BC to another, for example from G1 to G7.  This practice gives an approximate figure at best and is not reliable. 

Shooting a bullet that is marginally too long for the twist rate of the barrel may result in a bullet that is yawing at some point in the trajectory.  BC will also 'go away' as the bullet acts as though it is a much bigger diameter without increasing the length of the bullet.  The importance of matching a twist rate of a barrel to the length of a bullet and the distance at which one wants to shoot cannot be overstated.  Generally, a tighter twist rate results in a bullet that has a higher Sg (Gyroscopic Stability).  Use the guidelines or recommendation that I give. 

For exact numbers, shoot drop tables at the elevation and temperature for your location.