A bullet's BC is independent of atmospherics, the atmospherics are entered in a ballistics solver to obtain density of air and speed of sound not to change the BC of a bullet.
Please read this book if you are in doubt about the BC's independence of atmospheric conditions.
I am not saying that atmospheric conditions do not change the trajectory because it does because the denser the air is the greater the drag is, assuming both bullets are the same and have equal speeds.
