If I recall correctly, the CD (coefficient of drag) is the aerodynamic coefficient that is used to derive the BC. The G1 drag function is a very idealized shape to represent drag of a typical bullet. Later, other 'G values' were developed to refine the drag function of other types of projectiles (main point here is they were primarily centered around firearm projectiles). I believe it was the folks who developed Chairgun that went through the math and ballistics studies and determined none of those numbers really worked well to fit what was taking place with a diabolo shape, that was fairly common to airgun pellets (most closely matching the shape of a pellet like a JSB or H&N Baracuda). Thus they went through the effort to derive the math to mimic that diabolo drag function and store it as a new entity called 'GA' (the 'A' being selected to stand apart, and trigger thoughts of 'airgun' would be my guess).
I summarize this to point out that the CD is actually used in the mathematical derivation of the BC and is tied together to the shape of the projectile being classified-so the CD is linked to the BC mathematically. In other words, the same folks who developed the GA drag function for the diabolo pellet needed to (somewhere in the process) empirically (through testing-perhaps via a sort of wind tunnel measurment), or via modeling (CFD, perhaps), or theoretically (purely mathmatically) come up with the CD of a diabolo pellet shape. Thus the two are tied together and if you can't find GA, you most likely won't locate the CD for the diabolo shape either (because the two go hand in hand).
Does that help to understand a bit better?
Some of the newer slugs may actually have a better chance at being modeled with some of the G1, etc numbers because they mimic a typical firearm projectile more than a diabolo pellet shape.
Regarding software, you can download an awesome version of 'Chairgun Chairgun Pro' for either Mac or Windows straight from the Hawke website (Hawke is the company that developed Chairgun, or at least bought it and took it over from whomever did the original programming). Visit
https://us.hawkeoptics.com/chairgun.html and click the link to download for Windows for your free copy of the software. It really is a top notch piece of software-better on the desktop than on the phone for sure. There is a built in BC calculator with 'GA' (along with others) available. Plus you can print scope tapes, range cards (ballistics cards), etc and select any of the hawke scope to have the correct reticle displayed, etc. There is a ton of functionality in the software and it's terrific that its free to download and use as well.
Hope that helps somewhat?
Sean
