Range Report Trying to understand BC calculation formulas

[johnsopa]

I know software exists for calculating BCs, but I'm trying to understand the math behind it.

According to wikipedia (which we all know is the ultimate source for correct information, ha ha),

The variable information is here: http://en.wikipedia.org/wiki/Ballistic_coefficient#Bullet_performance

So, let's say that I have a bullet with measurable caliber and mass properties and that I also have a start and stop velocity and a known separation of the two velocity measurement points, how do I go from that data to the BC calculation?

I would appear that I need to calculate the drag coefficient, yes?

However, what's shown in the wikipedia info doesn't seem to match what I'm seeing in the JBM info. If you look here: http://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi

all that JBM requires is a start and stop velocity. No sectional density, etc.

I'm confused by what appear to be two completely different approaches to calculating BC.

Anyone interested in helping to clarify things?

 

[TiroFijo]

Re: Trying to understand BC calculation formulas

http://www.exteriorballistics.com/ebexplained/index.cfm

 

[johnsopa]

Re: Trying to understand BC calculation formulas

Thanks. It appears, according to this, that Sierra's method is to computationally iterate between narrowing BCs until a particular BC produces the measured final velocity, given a start velocity and various environmental factors.

(Source: http://www.exteriorballistics.com/ebexplained/5th/2311.cfm)

That tells me they aren't using any formulas but instead, are comparing results against either G1 or G7 data under the same condition.

I'm not sure what I think of this approach...

 

[jbmillard]

Re: Trying to understand BC calculation formulas

Take a look at the formulas here:

CD and KD

Look at the formula right before the "KD and CD" heading about half way down the page. As you can see, you can find the acceleration from the BC, the G function, density and gravity. My BC calculators iteratively integrate this equation to find the BC that matches the conditions you enter.

So yes, a formula is involved.

Brad