A little light reading for you Rob
Ballistic Coefficient - This is a number that relates to the effect of air drag on the bullet's flight and which can be used to later predict a bullet's trajectory under different circumstances through what are called "drag models." Technically a drag models applies only to a particular bullet, so using them to predict another bullet's performance is an approximation--but the results can be very close if the proper drag model is used. The most commonly used drag model is the G1 model (sometimes referred to--not really correctly--as C1) which is based on a flat-based blunt pointed bullet. The "standard" bullet used for this model has a ballistic coefficient of 1.0. A bullet that retains its velocity only half as well as the model has a ballistic coefficient of .5. The G1 model provides results close enough to the actual performance of most commercial bullets at moderate ranges (under about 500 yards) that it is commonly used for all commercial ballistics computation.
Note that there are two "standard" sets of meteorological conditions in common use. conditions" refer to an assumed used to standardize computations. The older one, is known as "Standard Metro" or "Army Standard" and the more modern "standard" is called the International Civil Aviation Organization (ICAO) standard. The characteristics of these two "standards" are listed below.
Standard Metro ICAO
Altitude Sea level (0') Sea level (0')
Temperature 59° F 59° F
Barometric pressure 29.5275" Hg 29.9213" Hg
Humidity 78% 0%
While they are similar, the different parameters do have a slight affect on calculations and in effect change the standard atmospheric density by about 1.8 percent. Under ICAO conditions the speed of sound 1116.5 f/s and under Standard Metro conditions it is 1120.27 f/s.
Since a quoted ballistic coefficient depends on atmospheric density, the same bullet has two different BCs depending on the conditions used. If a quoted BC based upon the "Standard Metro" conditions is used in a ballistics program based upon the ICAO standard the BC needs to be modified by multiplying it by .982. Conversely, ICAO based BCs need to be multiplied by 1.018. While this is a very small change and has little effect at short (under 600 yards) range it does have an effect at long ranges. The table below gives what various manufacturers use.