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.