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You Only Need to Know This If You Shoot Past 700 Yards!

 
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  #1  
Old 02-16-2013, 05:14 AM
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You Only Need to Know This If You Shoot Past 700 Yards!

Using The Right BC in Your Ballistic Application

A bullet has two (2) different BCs
values under the same drag function.

Remember, ballistic coefficient depends on
Air Density.

BC (Ballistics Coefficients) are either based
on "US Army" also known as "Standard
Metro" or on ICAO Standard Atmosphere
adopted by the US department of Defense in
1956. Here is where the two different BC values
come in.

Here is the catch. You need to know what
Standard Atmosphere your Ballistics
Application is referencing the BC on. Said
in different words, we need to know which of
the two BCs our ballistic application is
expecting us to provide, either the one
based on ICAO or the one based on Standard
Metro.

Under about 700 yards you don't have to
worrie about these differences, but other
wise we should probably be aware of them.

Example:

As most of you know, my favorite Ballistics
Application is ColdBore 1.0; this application
assumes that the BC that I'm entering in is
referenced to the ICAO Standard
Atmosphere.


BC Reference
Berger ICAO
Barnes Std Metro
Hornady Std Metro
Nosler ICAO
Lapua ICAO
Sierra Std Metro
GI APG ICAO

So, Berger and Nosler and Lapua bullet manufacturers their BC values are referenced to ICAO and that’s exactly the value that my application expects me to give it.

If, I use Hornady bullets with ColdBore, I need to modify their BC which is referenced to Std Metro to ICAO. To do this all I have to do is multiply the given BC by 0.982
The 0.338 Barnes 265 gr. Bullets is claimed to have a BC of 0.575; so I would modify it by:
0.575 x 0.982 = 0.565, and that would be my BC value to use in CB1.0

There it is, for this bullet in the G1 drag function you have two (2) different BC values, 0.575 and 0.565, we also know which one to use in this particular case.

Now, if the application is to use BC based on US Army “Metro Std”, then you would use Barnes given BC value with no modification.

Ballistics Explorer is one of the so many applications using the Metro Std. In this case, to use an ICAO based BC value, you would modify it by multiplying it by 1.018…
0.565 x 1.018 = 0.575-------

The difference can make 1.5 FT difference at 1500 yards…

How do I know what my Ballistics Calculator or Application uses?

Usually, when you run a Ballistics Application you would be able to see the input boxes populated by default values. Look at the atmospheric STD conditions and if you see this:

Altitude: 0 ft.
BP of 29.53 inches of mercury at a
Temperature of 59 degrees F
Humidity = 78 %

Then you know the application assumes the user to give it BC values based on “Army Standard Metro Conditions”

If you see this:

Altitude: 0 ft.
BP of 29.92 inches of mercury at a
Temperature of 59 degrees F
Humidity = 0 %

Then you know the application assumes the user to give it BC values based on ICAO

Good Shooting!

Note: Most of the information I got from CB1.0 User Manual.
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Our Lord Jesus said that as it was in the days of Noah and
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to this generation and we once more are rejecting it as was prophesied!!!

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  #2  
Old 02-16-2013, 06:19 AM
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Re: You Only Need to Know This If You Shoot Past 700 Yards!

Very good, thanks!!
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  #3  
Old 02-16-2013, 06:46 AM
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Re: You Only Need to Know This If You Shoot Past 700 Yards!

Wow! I did not know this, thank you sir!
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Old 02-16-2013, 07:09 AM
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Re: You Only Need to Know This If You Shoot Past 700 Yards!

Interesting stuff, and easy to see how you COULD make a mistake on the calculation, when entering in the PROVIDED BC info. Thanks for the update!
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Old 02-16-2013, 08:03 AM
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Re: You Only Need to Know This If You Shoot Past 700 Yards!

I use NF Ballistic software or perry systems. I shoot the 210 Berger and I enter Bergers Advertised BC with my NF . Do you know if this is correct ? Thanks.
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  #6  
Old 02-16-2013, 10:00 AM
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Re: You Only Need to Know This If You Shoot Past 700 Yards!

Your talking only about G1 BC figures correct?

Myself I use G7 and to my understanding it is for all intents and purposes a tiger of a different strip, but basically a better fit (base model) to modern long range bullets.

I'm not trying to hijack or confuse anyone so I think it should also be pointed out that.
The standard model projectile for a G1 BC is a flat based spire point

Where the standard model for G7 is a Boattail spire point.

Maybe Bryan Litz can weigh in on this, since he's my source of info on this matter.
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  #7  
Old 02-16-2013, 08:52 PM
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Re: You Only Need to Know This If You Shoot Past 700 Yards!

Quote:
Originally Posted by bigbuck View Post
I use NF Ballistic software or perry systems. I shoot the 210 Berger and I enter Bergers Advertised BC with my NF . Do you know if this is correct ? Thanks.
Friend, I have Exbal v6.5 and it requires BC values based on "Metro Std".
You need to modify the BC values from Berger and Nosler, but you don't from Sierra.

At the risk of repeating my self, any ICAO based BC values you modify by multiplying
by 1.018 which of course will produce a higher BC value.
__________________
-----------------------------

-----------------------------
HEBREWS 13:8
Jesus Christ the same yesterday, and to day, and for ever.

Our Lord Jesus said that as it was in the days of Noah and
also as it was in the days of Lot so it shall be in the days...
It's happening again!!! God sent to us His prophet, and His Word
to this generation and we once more are rejecting it as was prophesied!!!

---> As promised, God Sent His Prophet to us!
Reply With Quote
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