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G1 vs G5 calculating BC?

 
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Old 12-15-2008, 08:27 PM
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Re: G1 vs G5 calculating BC?

Lots of ballistic programs us G1 drag model as the basis for their calculations. The problem is the G1 drag model was developed over 100 years ago is a based on a 1 pound 1 inch round nose projectile. Modern bullets donít fit too well into this model built for a 1 pound 1" round nose projectile.

However, bullet manufactures use the G1 BC to advertise their bullets BCís. This is because G1 drag model produces higher BC's numbers and higher BCís sell more bullets. Bullet manufactures calculate the G1 BCís by a computer and most times out 10 wonít actually match the true BC of bullet out of your gun. So if youíre using the advertised G1 BC youíre probably already starting off with incorrect input data.

Modern bullets such as boat tails fit the G5 coefficient drag model and VLDís fit the G7 coefficient drag models. However the G5 and G7 coefficient drag models produce lower BCís but there ballistic calculations are very accurate, much more than the standard G1 drag model. Lower BC numbers won't sell as many bullets...marketing marketing marketing!

To correct and compensate for the short comings of the G1 drag model, ballistic programmers massage their G1 ballistic calculations. One way to do this is by using multiple BCís. However itís difficult to accurately predict or measure all the de-accelerating BCís. So a lot of time youíre guessing and that's not a very good way to get accurate results.

In the past these G1 massaged calculations were mediocre at best. Todayís ballistic programs have much better mathematical calculations and have good to very accurate calculations. However, the best method is calculate your bullets exact BC and match it the correct coefficient drag model. Or, at least convert the advertised G1 BC to the corrected G5, 6 or 7 depending on the bullet type and use the correct coefficient drag model on your ballistic program.

Hope it helps
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Old 12-16-2008, 12:46 AM
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Join Date: Mar 2007
Location: Montana
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Re: G1 vs G5 calculating BC?

Thanks for all the input guys. I like this tech stuff as well. This explains why the bc's for the GS bullets doesn't look good. They claim that theirs are measured on the G5 model. Run their #s on the ballistics program and the drops are very impressive. I will have to spend some more time playing w/ them.

Thanks again, Steve
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