Re: Ballistics Notation Question
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Crisisman</div><div class="ubbcode-body">I got my answer. Apparently they are velocity ranges. I'm no Aerodynamics expert, so I don't understand why drag changes at various velocities, perhaps with the sound barrier? Anyhow, now I know how to use the info. Thanks. </div></div>
This is why:
Drag varies greatly with velocity. The shape of that curve is standardized so that you can scale it to your bullet if you know it's BC, which is a scaling factor that is not supposed to change with velocity. This works well when the shape of the standard drag function matches that of your actual bullet well.
But the common G1 drag function is such a poor fit to the actual drag profile of long range (boat tail, large ogive) bullets, that the bullet makers have to correct for this by changing the BC as velocity changes. It's nothing more than a mathematical kludge.
The proper way to do it is to use a drag function that has the same shape as the drag profile of your bullet. For long range bullets, that generally means G7. This way, BC will not vary (significantly) with velocity. That's the way it's supposed to work. The wrinkle is that almost nobody publishes G7 BC's (which are not the same as the typically published G1 BC's). Bryan's book is the first major commercial effort to do so that I'm aware of.
So why not just do away with this standard drag function/BC business and just measure the drag profile of every bullet? It's insanely expensive. The military does that. That's why we know so much about the 168gr MatchKing, for example. They've studied the crap out of it.