Using The Right BC in Your Ballistic Application A bullet has two (2) different BCs values under the same drag function. Remember, ballistic coefficient depends on Air Density. BC (Ballistics Coefficients) are either based on "US Army" also known as "Standard Metro" or on ICAO Standard Atmosphere adopted by the US department of Defense in 1956. Here is where the two different BC values come in. Here is the catch. You need to know what Standard Atmosphere your Ballistics Application is referencing the BC on. Said in different words, we need to know which of the two BCs our ballistic application is expecting us to provide, either the one based on ICAO or the one based on Standard Metro. Under about 700 yards you don't have to worrie about these differences, but other wise we should probably be aware of them. Example: As most of you know, my favorite Ballistics Application is ColdBore 1.0; this application assumes that the BC that I'm entering in is referenced to the ICAO Standard Atmosphere. BC Reference Berger ICAO Barnes Std Metro Hornady Std Metro Nosler ICAO Lapua ICAO Sierra Std Metro GI APG ICAO So, Berger and Nosler and Lapua bullet manufacturers their BC values are referenced to ICAO and that’s exactly the value that my application expects me to give it. If, I use Hornady bullets with ColdBore, I need to modify their BC which is referenced to Std Metro to ICAO. To do this all I have to do is multiply the given BC by 0.982 The 0.338 Barnes 265 gr. Bullets is claimed to have a BC of 0.575; so I would modify it by: 0.575 x 0.982 = 0.565, and that would be my BC value to use in CB1.0 There it is, for this bullet in the G1 drag function you have two (2) different BC values, 0.575 and 0.565, we also know which one to use in this particular case. Now, if the application is to use BC based on US Army “Metro Std”, then you would use Barnes given BC value with no modification. Ballistics Explorer is one of the so many applications using the Metro Std. In this case, to use an ICAO based BC value, you would modify it by multiplying it by 1.018… 0.565 x 1.018 = 0.575------- The difference can make 1.5 FT difference at 1500 yards… How do I know what my Ballistics Calculator or Application uses? Usually, when you run a Ballistics Application you would be able to see the input boxes populated by default values. Look at the atmospheric STD conditions and if you see this: Altitude: 0 ft. BP of 29.53 inches of mercury at a Temperature of 59 degrees F Humidity = 78 % Then you know the application assumes the user to give it BC values based on “Army Standard Metro Conditions” If you see this: Altitude: 0 ft. BP of 29.92 inches of mercury at a Temperature of 59 degrees F Humidity = 0 % Then you know the application assumes the user to give it BC values based on ICAO Good Shooting! Note: Most of the information I got from CB1.0 User Manual.