Ballistic coefficient
Differences in BC claims for exactly the same projectiles can be explained by differences in the air density used for these BC statements or differing range-speed measurements on which the stated G1 BC averages are based. The BC changes during a projectile's flight and stated BCs are always averages for certain distances and speeds. Some more explanation about the transient nature of a projectile's G1 BC  during flight can be found at the external ballistics article. This article suggests that knowing how a BC was established is almost as important as knowing the stated BC value itself.