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Help with ballistics please.

 
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  #1  
Old 02-15-2011, 05:40 PM
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Join Date: Jun 2010
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Help with ballistics please.

Hiya Guys,

Can you correct me if I am wrong here, I have used the 87 grain V-Max bullet with varying successes. I find with my 20" 1 in 10 barrel that it will give me great accuracy at 100yds. The problem is as the bullet slows in flight. I use the following tools to check the ballistics.

Stability calculator. >>> Stability calculator

Bullet Length >>>>> Bullet lengths

As the bullet slows down the bullet becomes unstable and by using Nightforce Exbal you can soon see that the 87grain V-Max pushed through my rifle with a 1 in 10 twist barrel at a muzzle velocity of 3125 will start to become unstable at approximately 2730fps.



Whereas if I do the calcs on Berger 80 grain varmint they remain stable down to 1320fps and out to 700yds.



What are your thoughts ???

If I want to go long range how accurate are my figures and can they be relied upon ???
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  #2  
Old 02-15-2011, 07:40 PM
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Re: Help with ballistics please.

Help me understand how you calculated the point of unstability.
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Old 02-15-2011, 11:29 PM
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Re: Help with ballistics please.

ROBBO= I have shot both the 88 berger and the 87 horn bthp ; ( both bullets are longer than the vmax i believe) out of my 12 twist 243 . i have shot them out to 1000 yards and they did not become unstable. not only that they shot real good.
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Old 02-16-2011, 01:53 AM
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Re: Help with ballistics please.

What are you talking about?
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  #5  
Old 02-16-2011, 02:52 AM
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Re: Help with ballistics please.

Quote:
Originally Posted by learning View Post
What are you talking about?
Guys,

I am checking at what velocity the bullet becomes unstable on the bullet stability program here. >>>> JBM - Calculations - Stability

eg. If you check the data on the program above with 2 bullets.

87 grain V-Max = 1.037" long.
80 grain Berger Varmint = .925" long.

Add that data into the online software and play with the velocities and see where the bullets lose stability with a 1 in 10 twist barrel.

87grain V-Max starts to lose stability at 2720fps.
80grain Berger starts to lose stability at 1300fps.

I then look at the exbal data to see at what range the bullet reaches that velocity.

You will then see that the Berger bullets retains stability a lot further down range.

Do you understand what I am saying now ???

Or am I just totally wrong here ???

Last edited by robbobsam; 02-16-2011 at 03:17 AM.
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  #6  
Old 02-16-2011, 10:06 AM
KRP KRP is offline
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Re: Help with ballistics please.

Rotational velocity degrades at a much slower rate than forward velocity and that formula is for calculating stability at the muzzle not down range. I've shot 87 vmaxes to 1000 yards with a muzzle velocity ~200 fps less than yours.
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  #7  
Old 02-16-2011, 10:33 AM
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Re: Help with ballistics please.

Quote:
Originally Posted by robbobsam View Post
Guys,

I am checking at what velocity the bullet becomes unstable on the bullet stability program here. >>>> JBM - Calculations - Stability

eg. If you check the data on the program above with 2 bullets.

87 grain V-Max = 1.037" long.
80 grain Berger Varmint = .925" long.

Add that data into the online software and play with the velocities and see where the bullets lose stability with a 1 in 10 twist barrel.

87grain V-Max starts to lose stability at 2720fps.
80grain Berger starts to lose stability at 1300fps.

I then look at the exbal data to see at what range the bullet reaches that velocity.

You will then see that the Berger bullets retains stability a lot further down range.

Do you understand what I am saying now ???

Or am I just totally wrong here ???
I think it's me that's not very bright. Help me understand...

I used the JBM stability calculator and input the data from the 87g V-Max and it returned output data of 1.359. How do you relate this data back to velocity?

Seems inconceivable that the bullet would be unstable at 2720 (175 yards).
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