G7 velocity range measurement

[TC]

I notice the G7 for the Berger 185 for example is calculated from a starting velocity of 3,000 fps.

I can't imagine many are shooting it that fast and wonder if a more realistic/practical G7 would be produced by starting at a more reasonable/common velocity or doesn't that impact the final result ?

If I'm shooting it say at 2,500 would I be better off using the average of the G7 from there to sub 1.500 to give myself a more customised outcome ?

Thx

 

[Bryan Litz Ballistics]

The average G7 BC from 3000 to 1500 fps won't be much different than the average from 2500 to 1500 fps.

When using G1 BC's, you have to be very mindful of the exact velocity band, but the G7 tends to be pretty constant for all speed. You can use the 3000-1500 fps average for any supersonic trajectory and be good. It may begin to get shaky around transonic speed (under 1340 fps), but down to there it's OK

-Bryan

 

[TC]

Good to know.

Do you think we will ever see a new 'G' that incorporates 1500 down to say 750 / through the transonic range ?

 

[Bryan Litz Ballistics]

There are already many G-standards for various shaped bullets.

Trouble is, each bullet is unique, especially in it's transonic drag. No matter what G-standard you pick, it would only be an accurate representation for a small number of bullets.

For truly accurate drag modeling thru transonic speeds, you need 'custom drag models' for each bullet.

In addition to providing G1 and G7 BC's, Applied Ballistics also measures and provides custom drag models for bullets. The drag models are lengthy tables of CD vs. Mach, and you can't really tell anything by looking at them like you can with BC's. The tables are read by a ballistic solver when modeling trajectories, rather than referring to a G standard.

You can use custom drag models thru the AB apps, AB Kestrels, and AB Analytics for PC.

Here's the instructions for AB Mobile which talks more about custom drag models: http://appliedballisticsllc.com/AB_app.htm

-Bryan

 

[TC]

Thanks for that. Very helpful.