Ballistic Coefficient and Drag Function

[Laphroaig]

A recent thread gave a link to the Hornady Ballistics Calculator. I was interested because I came up with a new load for my 30-06 vintage sniper rifle and wanted to get it on paper for 300 at my 200 yd. range. A 600 yd. come up will also be handy.

Nonetheless, I plugged in my data for 175 SMK's and got confirmation that 3 MOA between 200 and 300 was pretty close. While researching the ballistic coefficient on the Sierra site, they give multiple coefficients based on velocity. Other bullet manufactures only give one number, which I think is based on the highest velocity. I trust the Sierra numbers, and they apply to me more because I never seem to achieve the max velocity in my old -06's. Does anyone know the science behind the variance between ballistic coefficient and muzzle velocity?

Also, there is a field for "drag function" on the Hornady Calculator, which indicates either G1 or G7 depending on bullet, but never explains what it means.

Thanks.

Laphroaig

 

[sirgilligan]

I wrote the ballistics calculator found in my iOS apps.

When only one BC and velocity is given, the velocity is most likely the muzzle velocity and the drag method / function / model is most likely G1.

There are several approaches to calculating the bullet trajectory, and these methods are associated with different methods, and the methods have names like G1, G7, etc.

If no designation is given for the BC listed then it is typically a G1 ballistic coefficient. You have to use the proper method when running a ballistics calculator.

Some quick searching found these:

http://www.jbmballistics.com/ballistics/topics/topics.shtml


This shows some of the differences between the drag functions and maybe answers your question.

http://www.jbmballistics.com/ballistics/downloads/text/olin.txt


http://www.shooterscalculator.com/notes/drag-functions.htm

http://www.exteriorballistics.com/ebexplained/articles/the_ballistic_coefficient.pdf

http://www.frfrogspad.com/extbal.htm

 

[taliv]

it is pretty much rocket science and will take a lot to explain. a few minutes reading the wikipedia entry for ballistic coefficient would be better than random anecdotes here.

the very short story is that in theory, G1 is a drag function for a flat base profile (like a FMJ pistol bullet) and G7 is a drag function for a boat tail (e.g. rifle bullet), maybe VLD i can't remember. Anyway, it's not quite an arbitrary index, but it really is dependent on the algorithm used in the calculator and there are different ones (e.g. point mass). so while some calculators use either or recommend g7, others that are very well respected like FFS, actually use G1.

the other thing to understand is the difference between calculated and measured. Sierra's advertised BCs (last i checked) were calculated based on the shape of the bullet, where brian litz uses a doppler radar to measure the actual drop of bullets in flight over long distance and publishes his findings in his book. he also works for berger so the numbers berger publishes are measured by litz.

in the case of FFS, they recommend calibrating both the velocity and bc with internal calculators and not using any published data

 

[Laphroaig]

Thanks guys. I did read through a couple of those links and at least figured out that the G1 drag coefficient is the one to use with the published BC's. It is rocket science!

I'm just trying to get on paper anyway so can't really justify buying a book or special software.

Thanks again.

Laphroaig

 

[taliv]

most companies publish both G7 and G1. it will say right next to it which is which. it's not clear to me why that would be confusing.

if you're just trying to get on paper, from a 100 yard zero,
200 3 moa
300 6 moa
600 15 moa
will probably be within a foot. use a big sheet of paper