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calculating bc

 
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  #8  
Old 12-29-2010, 05:44 PM
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Re: calculating bc

Kevin,

I am glad there are nerds like you to keep us all straight. You should call wikipedia and correct them.

Arrow,

Check out this link Precision Ballistic Coefficient Estimator as another source.

Good luck!


Ed
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  #9  
Old 12-29-2010, 05:48 PM
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Re: calculating bc

You can start with your bullet maker's BC, as correlated with any other sources you can find.
This is often 'rough' because they are not generally qualified.
For example, the maker might list a BC of .460 for your bullet, but without mention of velocity or atmospheric standard(much less bullet lot#), this value was based on. It was probably estimated through math rather than field validated which could introduce a small error as well.
So for you it's a generalization about as accurate as you could eyeball yourself..
But it's a start, and Berger and Lapua are advancing this process for us. Someday it won't be so difficult. Berger has lately listed BCs based on average velocities common to a bullet's use, and this is about the best information available off a bullet box.
For long range bullet's, Bryan Litz(with Berger) has measured, calc'd & tested, and provided results in a very good book: Book
Muzzle velocity is needed for sure to convert G1 BCs to G7, so plan on doing so AFTER load development across a chronograph.

To correct your BC, you could use 2 chronographs to get near & far velocities for calculating BC. This has also been done with an Oehler setup that measures 'time of flight' with a triggering chrono + downrange microphones.
Or you can shoot several distances and tweak your BC inputs to ballistic software for the closest match at all three distances. But counterintuitively, this is actually the least accurate method.
Exbal has a BC validation screen for this, and LoadBase & Pejsa's provide a process for this method as well as the others.

For a 175smk:
Sierra list's it .505@2800fps+
Sierra Bullets - The Bulletsmiths
I believe Sierra bases this on Standard Metro atmosphere, and converted to ICAO atmosphere this BC becomes .496
Your version of Exbal is one atmosphere standard or the other(or another) so be sure to understand this during use.
An ICAO G1 BC of .496 @ 2800fps converts to a G7 BC of .248 with same qualifiers.
You can do this here: JBM - Calculations - Drag Function Conversion
Bryan concluded a little lower G1/G7 BCs for this bullet in his book. He also provided averaged BCs throughout 1500-3000fps as .475/.243
(I hope to morally provide this info, and not burn in hell)
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  #10  
Old 12-29-2010, 08:23 PM
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Re: calculating bc

Quote:
Arrow,

Check out this link Precision Ballistic Coefficient Estimator as another source.

Good luck!
I have used that link to calculate the BC on some of the 257 cal Wildcat bullets and then tweaked the answer to account for the rebated part. It comes very close. Of course trying to measure meplat diameter is about as goofy as figuring out exactly where to measure the boattail to get its diameter. None the less the link will get you real close to a good G1 BC and then you extrapolate a G7 from other known bullets and away you go to the range to verify it.
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  #11  
Old 12-29-2010, 08:55 PM
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Re: calculating bc

Holy crap I should have paid more attention in my math classes!!
I'm just happy there are progremmes like exbal & bergers new program.
The interesting thing when I plug all the numbers from my Exbal print off witch is pretty darn close out to 1000yards and put into Bryans program I have to tweek the bc from .750 wildcat 169.5 to .770 to come out with the same drops time of flight ext. but the extra .20 bc adjustment puts every thing within 1/2 foot per second @1000 energy with in 1lbs time of flight nono second off I had goodgrouper do the load work up on Exbal but I used Bryans program on my last hunt adjusted every thing to 10 yard increments got on google earth to make sure elevation was correct. had to guess what temp would be but it paid off our group killed three antelope in 1 hour & 45 minuets fun but our trip was over to fast.
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  #12  
Old 12-29-2010, 10:45 PM
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Re: calculating bc

Well i did something interesting today... i took a laptop and a microphone into the trench below the target whilst my friend shot from 1000yds away...

I recorded the soundtrack using "audacity" and then once id collected the data, i used the spectogram waveform to isolate the time difference of the sonic crack of the bullet flying overhead from the muzzle report a short time later.

Using the Time of Flight in the ballisitcs calculator, and the time it takes sound to travel 1000yds @ 26deg C, i could accurately predict his muzzle velocities to less than 1% error assuming we had an accurate BC. I recorded a string of 5 shots and even gave him the extreme spread for his load!

So, Using similar ideology in conjunction with an accurate chrony at the muzzle, we could reverse calculate a G7 BC in the same way. Beleive it or not, it is surprisingly accurate and you can clearly record time differences of .0005 seconds resolution.

Heres an article that explains the idea... - http://arxiv.org/ftp/physics/papers/0601/0601102.pdf

Last edited by groper; 12-29-2010 at 10:50 PM.
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  #13  
Old 12-29-2010, 10:55 PM
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Re: calculating bc

Amazing idea!!!!!
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  #14  
Old 12-29-2010, 11:49 PM
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Re: calculating bc

I read the exact same paper. That's how I plan on measuring our bullets BC's.

In fact, I nudged a kid at school into doing this for his science project. Audacity is free and most people already have a laptop.

I'd just use the time of flight and JBM's Ballistic Coefficient (Time) calculator HERE to get the BC's from the time of flight data from the sound card.

We'll see how it matches with my drop data. I'm also going to use Adobe's Sound Booth to see if the time measurements differ.


Quote:
Originally Posted by groper View Post
Well i did something interesting today... i took a laptop and a microphone into the trench below the target whilst my friend shot from 1000yds away...

I recorded the soundtrack using "audacity" and then once id collected the data, i used the spectogram waveform to isolate the time difference of the sonic crack of the bullet flying overhead from the muzzle report a short time later.

Using the Time of Flight in the ballisitcs calculator, and the time it takes sound to travel 1000yds @ 26deg C, i could accurately predict his muzzle velocities to less than 1% error assuming we had an accurate BC. I recorded a string of 5 shots and even gave him the extreme spread for his load!

So, Using similar ideology in conjunction with an accurate chrony at the muzzle, we could reverse calculate a G7 BC in the same way. Beleive it or not, it is surprisingly accurate and you can clearly record time differences of .0005 seconds resolution.

Heres an article that explains the idea... - http://arxiv.org/ftp/physics/papers/0601/0601102.pdf
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