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Determining ballistic coefficient...
Help / Support / How-to's / Discussion related to PointBlank Software and External Ballistics in general
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kudukid
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PostPosted: Mon Nov 21, 2011 8:22 am    Post subject: Determining ballistic coefficient... Reply with quote

Can PointBlank compute ballistic coefficients given data from muzzle and downrange chronograph recorded speeds?

I am skeptical that this is being done at the various bullet manufacturers facilities.
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Ominivision1
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PostPosted: Mon Nov 21, 2011 11:47 am    Post subject: Re: Determining ballistic coefficient... Reply with quote

I have never tried what you are describing but Dallan will be along to give you a heads up on your question.

I also take the bullet coefficients from the manufacturers with a grain of salt so I use this calculator.

www.tmtpages.com/calcb...calculator

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DallanC
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PostPosted: Mon Nov 21, 2011 12:23 pm    Post subject: Re: Determining ballistic coefficient... Reply with quote

kudukid wrote:
Can PointBlank compute ballistic coefficients given data from muzzle and downrange chronograph recorded speeds?

I am skeptical that this is being done at the various bullet manufacturers facilities.

Yes. PointBlank will compute this if you know muzzle and 100 yard velocities. It basically runs the computations over and over internally adjusting the BC until it finds the closest match to the velocites given. Its pretty accurate.


-DallanC
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kudukid
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PostPosted: Mon Nov 21, 2011 1:48 pm    Post subject: Re: Determining ballistic coefficient... Reply with quote

Thanks...any idea where to look for this in the program and how to apply it? I'm new to PointBlank.

DO'C
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Fireman_DJ
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PostPosted: Mon Nov 21, 2011 4:49 pm    Post subject: Re: Determining ballistic coefficient... Reply with quote

At the top, hit Ballistics. Then on the bottom tabs hit Misc. Then it's the first button.

Note, it says to use the muzzle and 100 meter velocities. on the screen, but when you click the button it'll tell you either yards or meters depending on your unit of measurement settings.
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kudukid
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PostPosted: Mon Nov 21, 2011 9:41 pm    Post subject: Re: Determining ballistic coefficient... Reply with quote

Many thanks - should have second Chrony here on Thanksgiving day.
Don
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Fireman_DJ
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PostPosted: Mon Nov 21, 2011 10:15 pm    Post subject: Re: Determining ballistic coefficient... Reply with quote

Why two chronys? They might read differently anyway.

I assume you're hand-loading, so make sure you load a very precise batch (weight every charge of powder), then fire 10 shots with the chrony at the muzzle. Get the average value, then move the chrony and repeat at 100 yards/meters.

Take the two average values and plug them into PB.

It'll give a more accurate reading for the BATCH of pills your trying. Some may have a slightly better BC then others in the same batch.

Averages are your friend.
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SyberShooter
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PostPosted: Tue Nov 29, 2011 8:57 am    Post subject: Re: Determining ballistic coefficient... Reply with quote

.... as are standard deviations
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Ghost
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PostPosted: Tue Nov 29, 2011 11:01 am    Post subject: Re: Determining ballistic coefficient... Reply with quote

If not know the 100yds velocity, what then? Cause can't let the program calculate something if not can fill in the BC. And only can find out the 100velocity after that. So?? Confused
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Ominivision1
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PostPosted: Tue Nov 29, 2011 11:37 am    Post subject: Re: Determining ballistic coefficient... Reply with quote

Ghost wrote:
If not know the 100yds velocity, what then? Cause can't let the program calculate something if not can fill in the BC. And only can find out the 100velocity after that. So?? Confused

If you don't have access to a chrony, click the link on my post above and enter the data, at the top, the left side is for cast bullets and the right side is for jacketed bullets. Select what kind of bullet you have in the appropriate columns and enter data in the boxes and it will calculate (approx) BC's of given bullets.

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Ghost
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PostPosted: Tue Nov 29, 2011 4:47 pm    Post subject: Re: Determining ballistic coefficient... Reply with quote

A nice one to use too. Thx Ominivision.


Ghost
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slimjim
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PostPosted: Tue Nov 29, 2011 10:05 pm    Post subject: Re: Determining ballistic coefficient... Reply with quote

From what I've read, velocity measurements at 100 yards for deriving bullet BC is not very accurate. It's more accurate if you use a velocity closer to 300 yards. I've done this a couple of times and you need an accurate rifle you have confidence in to shot at your chrony at that distance. Of course, if you rifle isn't accurate, having a good BC value is irrelavant.

You shouldn't need a special funciton to derive BC. Just enter the data into the trajectory/ballistic caluclater with your guess at BC. Then adjust the BC value until the velocity at the range you took the chrony at match. I did this last weekend for a Barnes .223 62gr TTSX. I quess a BC of .3 and adusted it down to .292 which is where the velocity I had at 250 yards matched.

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chambered221
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PostPosted: Wed Nov 30, 2011 8:48 am    Post subject: Re: Determining ballistic coefficient... Reply with quote

Some added important info when dealing with Ballistic Coefficients !!!

Ballistic Coefficient - A comparative number to judge one bullet to another as to it's ability to travel through air and retain velocity.

First and foremost, The manufactures published numbers are arrived at by means of a mathematical calculation. In other words they are a pencil and paper numbers that only hold true in a perfect world.
This is one of the reasons manufactures don't always list the info and why so many re-loaders don't trust the numbers.

One aspect that can explain this is stability. Simply shooting identical bullets through barrels with different twist rates can change the BC of a given bullet.
When a bullet leaves the muzzle it is at it's most unstable point. The amount of time it takes to stabilize the bullet will change the BC of the bullet.
So once again, as it so often happens in the world of re-loading, variables come into play (as you can imagine they are numerous) and changes things. These changes can be small and insignificant to most hunters but they can also have huge and dramatic results to long range shooters.

The muzzle velocity is an important aspect of BC. A bullet shot from a .308 Winchester will not have the same BC as an identical bullet shot from .300 Weatherby. This is because the lower starting velocity puts the bullet into a different operating parameter.

BC from identical bullets can also change, imperfections in the bullet, tip damage and air disturbances can also create changes. Keep in mind the most important aspect to determining BC is the Time Of Flight. Change TOF and BC changes with it.


This brings us to another important point. When determining BC by means of 2 chronographs it is imperative to know the exact distance ( needs to be measured ) between the two points. If the proper distance is not used in the calculation the BC will be incorrect due to the TOF calculations.
The 2 chronographs also need to read to about 1fps of one another. Set them up in a over lapping manner to determine consistency between the two. Again TOF calculations will be incorrect if there is too much variance.

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Ominivision1
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PostPosted: Wed Nov 30, 2011 9:07 am    Post subject: Re: Determining ballistic coefficient... Reply with quote

Good point Chambered, further reading from Sierras website is HERE

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chambered221
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PostPosted: Wed Nov 30, 2011 11:27 am    Post subject: Re: Determining ballistic coefficient... Reply with quote

Excellent add on !!!

IMO Sierra has led the way with their BC research and listing of data.



Point #1 says it all !!! Sniper

Point #2 can be argumentative....... This is basically due to opposing sides not being on the same page. Here again the paper calculations don't deal with air resistance or air density. So on paper it's a constant as opposed to real word cause and effect.

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