Well, that equation is the one manufacturers use to ESTIMATE a bullets BC. The "form factor" variable is the one where manufacturers fudge. It would give a valid relative comparison between two bullets of identical shape, but the BC number won't necessarily conform to real world performance. You have to shoot the bullet to get an accurate MV and another accurate velocity reading downrange (at several distances if using the G1 standard), then, adjusting for atmospheric conditions, the average G1 BC can be calculated. If one uses the G7 standard with boattail bullets, the BC will remain much more constant over the entire velocity range, but the BC number will be smaller compared to the G1 BC. The G1 BC, when applied to bullets with a longer than 2 radius tangent ogive, secant ogive or boattail (which is most of the bullets fired from modern CF rifles), will change (decrease) as the velocity decreases. So a spitzer BT bullet at 3000 fps might have an actual G1 BC of .500, but at 2500 fps the BC may only be .450. At 2000 fps the BC might only be .375. If you use the .500 BC number (which is probably the number published by the manufacturer) in a ballistics calculator to calculate the elevation for a long range shot, you would likely miss low because the G1 BC doesn't remain constant as the bullet loses velocity. The G7 standard is much better for most rifle bullets because the drag model closely matches the G7 standard bullet, meaning that the BC changes very little if at all over the entire flight of the bullet. But manufacturers are reluctant to switch to it because the G7 BC number itself is far smaller than the G1 BC (for example, a Berger .308cal 185gr VLD has a G1 BC of 0.549, but a G7 BC of 0.281, but the G7 BC will give much more accurate to real world results when used with a ballistics program like JBM).

Mike