Hi,
When you use an exterior ballistics calculator program (there are a number of them, many free, on the Internet. I've used one called PointBlank for several years, available here if you want to give it a try: http://www.huntingnut.com/index.php?name=PointBlank ), it'll want a few bits of info to do its calculations. Included will be velocity (obtained from your chronograph or estimated from book numbers), bullet weight and BC (it may ask for more.)
As you've seen, the bullet's mfr has often figured out the BC for you. I don't remember the exact details, but they kind of reverse engineer it from data measured on the range--velocities and bullet drops at various distances. Sometimes the program will include a database of common bullets, so you can plug in the catalog number and it will look up the BC for you. With the one I've used, you can "add a bullet" for future use so I'd plug in the numbers the mfr provided.
That number works within the program's formulas (algorithms if you prefer) to help determine the downrange behavior--velocity and bullet drop--you'll want to know for sighting in and other purposes. The larger the number, the better (both faster and flatter, longer) the bullet flies over a given distance.
Rick C
