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Help with ballistics please.

 
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  #8  
Old 02-16-2011, 10:42 AM
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Re: Help with ballistics please.

Quote:
Originally Posted by trebark View Post
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).
Hiya Trebark,

Thank you for your time. If you play with the JBM stability calculator, add the correct bullet length and barrel twist, then keep reducing the velocity of the bullet you will see that when you reduce the velocity to below 2738fps it goes from green to yellow indicating that it starting to lose stability. Therefore I am assuming that the 87 grain V-Max bullet in .243win is unstable with that barrel twist of 1 in 10 at less that 2738fps.

I then run exbal, adding all the relevant data relating the Bullet velocity, BC sight in height etc...etc...etc... and you will see you are only 175yds away when the velocity of the bullet has reduced from muzzle velocity to the unstable velocity of 2738fps.

Am I making that clear ???

If not I will try again.
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  #9  
Old 02-16-2011, 10:43 AM
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Re: Help with ballistics please.

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Originally Posted by KRP View Post
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.
What is your barrel twist rate though ???

Is it 1 in 10, or 9 or 8 ???

I have just ran the same calculations with a 1 in 9 and a 1 in 8 twist barrel and as you say it will stay stable for up to 1000yds.

Last edited by robbobsam; 02-16-2011 at 10:48 AM.
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  #10  
Old 02-16-2011, 11:13 AM
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Re: Help with ballistics please.

Generally if a bullet is stable when fired it will remain stable until it reaches the transonic zone somewhere around 1250 fps.

edge.

PS Make sure you calculate using the temperature you plan to hunt with as the air density may take you from stable to unstable if you are on the edge!

Last edited by edge; 02-16-2011 at 11:18 AM.
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  #11  
Old 02-16-2011, 11:22 AM
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Re: Help with ballistics please.

Quote:
Originally Posted by robbobsam View Post
Hiya Trebark,

Thank you for your time. If you play with the JBM stability calculator, add the correct bullet length and barrel twist, then keep reducing the velocity of the bullet you will see that when you reduce the velocity to below 2738fps it goes from green to yellow indicating that it starting to lose stability. Therefore I am assuming that the 87 grain V-Max bullet in .243win is unstable with that barrel twist of 1 in 10 at less that 2738fps.

I then run exbal, adding all the relevant data relating the Bullet velocity, BC sight in height etc...etc...etc... and you will see you are only 175yds away when the velocity of the bullet has reduced from muzzle velocity to the unstable velocity of 2738fps.

Am I making that clear ???

If not I will try again.
Ok - so the 'color' of the output has something to do with the stability. But how do you utilize the data (1.359)?

Instability at 175yards really seems inconceivable. Berger recommends a 10 twist for their 88 grain bullets.
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  #12  
Old 02-16-2011, 11:32 AM
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Join Date: Jun 2010
Posts: 158
Re: Help with ballistics please.

Quote:
Originally Posted by trebark View Post
Ok - so the 'color' of the output has something to do with the stability. But how do you utilize the data (1.359)?

Instability at 175yards really seems inconceivable. Berger recommends a 10 twist for their 88 grain bullets.
If you look on the bottom of the page here >>>> JBM - Calculations it tells you to ensure the reading is between 1.3-2.0 for stability.

The 1.359 is just within the parameters.

If the number goes below the 1.3 it turns yellow which indicates the bullet is unstable.

At the velocity that the reading falls below 1.3 is the velocity that I look up on exbal and from that I then look across the exbal chart to give the range.

I am trying to get a better understanding of ballistics and how a bullet behaves in flight.

It appears to me that some bullets remain stable in flight longer and as such will give tight groups at longer ranges. I think this may have somthing to do with it.

Last edited by robbobsam; 02-16-2011 at 11:35 AM.
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  #13  
Old 02-16-2011, 11:38 AM
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Posts: 208
Re: Help with ballistics please.

robbobsam, read KRP's reply. Don't worry about stability calculations. Shoot to verify, even the 1250fps rule does not always apply. Search the net for "shooting a 223 to a mile." That blows the myth of 1250fps out of the water. Some bullet designs will destabilize when they go subsonic, but that is a general rule, not the golden rule. If you have the right set up you can shoot accurately a very long way. Every rifle bullet combination is unique so you have to shoot to verify. I shot a 1-14" twist 223 with a 75grn bullet that was keyholing at 50 yards. At that distance it was going way faster than 1250fps. I then shot that same bullet in a 1-9" twist and was grouping at 1050 yards no problem. At that range, in the right conditions the bullet was subsonic and still staying stable.
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  #14  
Old 02-16-2011, 12:09 PM
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Re: Help with ballistics please.

Quote:
Originally Posted by straightshooter View Post
SNIP.

Search the net for "shooting a 223 to a mile." That blows the myth of 1250fps out of the water.

SNIP.
FYI, I did NOT say that it WILL destabilize below 1250 fps, I said that generally if fired it is stable then it will stay stable down to 1250 fps.

edge.

OH, it is not a MYTH that there is a transonic zone that can cause instability...that seems strange to describe it that way!
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