Quote:
Originally Posted by jonoMT
Bryan,
I notice in the example you have @ http://www.appliedballisticsllc.com/...ookexample.pdf that the G7 BC has very little variation among velocities vs. the G1 BCs. Is that common? If so, it would seem to make even more sense to use the G7 BC for my ballistics calculations.
Thanks,
Jon

The G1 reference projectile is a flat base short ogive spitzer. The G7 reference eprojectile is a much more aerodynamic long ogive boattail bullet. If you're shooting 100 yard benchrest the popular bullets will be a better match for the G1 model. For 1000 yard plus shooting where wind deflection is the dominant source of shooting error most shooters chose long ogive boattails (VLDs) and those fit the G7 model better.
Why do most bullet manufactures only publsh the G1 BC number even for low drag boattails? Simple. Of all of the G() models the G1 reference projectile has the highest drag, therefore the G1 numbers are numerically higher. Other things being equal higher BC bullets sell better. Apparently the manufacurers think buyers are so dumb they'd go by the number alone. That may be true.
You cannot convert a G1 to a G7 BC by applying a constant or even a fixed function to the numbers. The relation of a bullets G1 and G7 BC is vs velocity changes with bullet shape.
Ideally the whole concept of BCs would be dropped and drag functions vs velocity provided for each bulllet instead. That would be very unwieldy to use with lookup tables and would require computer databases. It would give better trajectory predictions in todays world of personal computers. Even drag functions don't fully describe a bullets trajectory Ther are still stability factors vs velocity vs spin rate and second order effects beyond that.
WIll such databases every be generated and made available for download or sold on mass storage media by bullet manufacturers? Maybe if it imporoves sales (or more important, profit) but it takes considerable effort to measure or calculate the data accurately. Only a small part of the shooting community would care. Particulary in competitive shooting knowing a bullet's exact trajectory doesn't help much in trying to shoot smaller goroups or higher scores. With most manufacturers we're lucky to get a single G1 BC value for each bullet which is reasonably close at normal muzzle velocities. For most shooters that's good enough.
This website has drawings with the dimensions (in calibers) of the "G" reference projectiles:
http://www.frfrogspad.com/drgshape.htm