Sorry, I ran across this thread, and couldn't help the necropost.
Ballistics is a fascinating subject that has had my attention for years. More recently (well, a while ago now, again) when I used to do some hobby programming, I coded a ballistics simulator model. This is the basic one, here.
I had later made a better version, with a GUI coded with the Allegro API, which also calculated lateral displacement through gyroscopic precession of the spinning shell, too, but goodness knows where that is now. Lost on some HDD, I wonder. I had a bit more time, in those days, and was going to integrate it into a naval warfare sim I had in mind. LOL!
Anyway, here's the blog on its development. Note, I was still using lookup charts for the atmosphere and transonic characteristics of the shell, at this stage.
If you do take a look, I welcome comments, corrections and opinions. One day I'll do some coding again, I guess, so everything would be valid.