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# Correct Form Factor to use

#1
02-05-2009, 10:34 AM
 Bronze Member Join Date: Dec 2007 Location: NC Posts: 77
Correct Form Factor to use

I just read Bryan Litz's "What's Wrong With 30 Caliber" article, and noticed that with heavy bullets/VLDs he said that they more closely matched the G7 form factor. How do I figure out which form factor I need to use when running trajectories through "JBM Calculations." I used G1 & G7 on the JBM page, and got dramatic differences. Any help out there.

Thanks,
Shane
#2
02-05-2009, 04:06 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,975
Re: Correct Form Factor to use

Outside of word of mouth, the only way to know which drag model(ratio) fits best is through testing. If Brian says G7 works better, I tend to believe him.

Keep in mind though, you need to use different BCs for different drag models to produce the proper ratio and resultant drop calcs. FF is Cd for your bullet divided by Cd from a test bullet at any given Mach#. In this case, the G7 test bullet.. So you must enter G7 BC for your bullet to compare, if using G7 drag as a basis.
A 'starting point' for this is: G7BC = G1BC / 2.027176 That should get ya close.

Now keep in mind that local BC is different than std BC due to atmospheric conditions. So don't ignore these on entry of your field results for comparison.
#3
02-09-2009, 02:13 PM
 Bronze Member Join Date: Dec 2007 Location: NC Posts: 77
Re: Correct Form Factor to use

So, if I have a (published) BC for a 208gr Hornady A-Max of .648, then I need to divide by 2.027176, which = 0.31966. Then plug that into the ballistics calculator as my BC, along with the G7 FF, and current atmosphereic conditions?
#4
02-09-2009, 03:54 PM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,975
Re: Correct Form Factor to use

yes, that's right

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