Let me see if I can illustrate this. The drag of the bullet (using the ballistic coefficient
and drag function model) is proportional to the
drag function divided by the ballistic coefficient (among other things).
This means as the BC goes up, the drag drops. Since you have two numbers
to play with, the drag function value and the BC value, there are many
combinations that give you the same drag value. For example, you can use
the G1 drag function and a G1 BC or you can use the G7 (or G5, or G8...)
drag function and a G7 BC. As long as you use the BC developed for the
drag function you get the right answer -- that's the key. Look at the
following data calculated for the 210 grain Berger VLD:
Vel. G7 G1
1500 0.327 0.590
1600 0.320 0.600
1700 0.317 0.612
1800 0.316 0.621
1900 0.316 0.626
2000 0.316 0.630
2100 0.316 0.633
2200 0.317 0.636
2300 0.318 0.638
2400 0.319 0.640
2500 0.321 0.643
2600 0.322 0.647
2700 0.324 0.651
2800 0.326 0.656
2900 0.328 0.663
3000 0.330 0.670
3100 0.332 0.677
3200 0.333 0.686
3300 0.335 0.695
3400 0.337 0.705
3500 0.338 0.715
It is two ballistic coefficients as a function of velocity for two
different drag functions. If the drag functions fit the bullet perfectly,
the number would the same for all velocities -- that's the point of
the ballistic coefficient -- one number instead of having to remember
and implement many different coefficients.
Now when you look at the data above, which coefficient comes closer to the
ideal, the G7 or the G1? Obviously the G7 fits better because it varies
from 0.327 to 0.338 (difference of 0.011) compared to the G1 change
of 0.125, more than 10 times as much.
Now ask yourselves which BC the bullet manufacturers would rather list.
Would they rather publish 0.330 or 0.670? Of course they're going to
publish the 0.670 because it sounds better and many people don't
understand what it all means. So we're stuck with multiple ballistic
coefficients as a function of velocity to make up for the short comings
in the drag function. We could easily use a single number if we used
the G7 drag function. To make it worse, many programs don't (or didn't)
allow you to pick your drag function.
So, if you're using BCs published by Sierra, use the G1 drag function
or you'll get huge errors.
Brad
P.S. I would assume the pressure is atmospheric pressure used in the
air density calculation. Now whether it is absolute or corrected, I
couldn't tell you.
