Help with ballistics please.

Discussion in 'Long Range Hunting & Shooting' started by robbobsam, Feb 15, 2011.

  1. robbobsam

    robbobsam Well-Known Member

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    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.

    [​IMG]

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

    [​IMG]

    What are your thoughts ???

    If I want to go long range how accurate are my figures and can they be relied upon ???
     
  2. trebark

    trebark Well-Known Member

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    Help me understand how you calculated the point of unstability.
     

  3. roninflag

    roninflag Well-Known Member

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    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.
     
  4. learning

    learning Well-Known Member

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    What are you talking about?
     
  5. robbobsam

    robbobsam Well-Known Member

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    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: Feb 16, 2011
  6. KRP

    KRP Well-Known Member

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    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.
     
  7. trebark

    trebark Well-Known Member

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    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).
     
  8. robbobsam

    robbobsam Well-Known Member

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    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.
     
  9. robbobsam

    robbobsam Well-Known Member

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    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: Feb 16, 2011
  10. edge

    edge Well-Known Member

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    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: Feb 16, 2011
  11. trebark

    trebark Well-Known Member

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    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.
     
  12. robbobsam

    robbobsam Well-Known Member

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    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: Feb 16, 2011
  13. straightshooter

    straightshooter Well-Known Member

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    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.
     
  14. edge

    edge Well-Known Member

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    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!