Does bullet stability drop of with velocity??

Discussion in 'Long Range Hunting & Shooting' started by proload, Jun 13, 2011.

  1. proload

    proload Well-Known Member

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    Hi there,

    I'm developing a hunting load (up to red deer sized game) in my 243 Win with the Swift Scirocco 90 gr (My barrel twist 1 in 10") and I think that I am seeing evidence of "very slight" instability (yawing) by analyzing the holes in my targets. I can't see how I will get good accuracy if the bullet is unstable, so I got on to a bullet stability calculator. The best result from the calculator is that the bullet is unstable/marginally stable (1 in 10", 3250 fps, Projectile Length 1.135"), and I am wondering if it's even worth trying to tweak my load, or if I'm on a hiding to nothing.

    Everyone that I speak to says that this projectile should stabilize, but the maths/science says to me that I'm wasting my time?

    I guess what I'm asking is this:

    1. Am I wasting my time?
    2. Will the bullet become even less stable in flight (as the velocity drops off) or not.
    3. Will I be better off using a 85gr Nosler Partition? The stability calculator claims that this bullet is better suited to my set up.

    Cheers
     
  2. justgoto

    justgoto Well-Known Member

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    I get that a lot; but have found the calculators to be correct.

    Using loads I've developed with bullets I can't stabilize, I get 1/4" larger groups at 400 yards, than with bullets I can stabilize. I don't see it affecting accuracy, but energy at extended ranges.

    I use the longest bullet the calculator suggests.
     

  3. proload

    proload Well-Known Member

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    Thanks for that,

    So firstly, do you think that the projectile will become more and more unstable as it flies, or do you think that it won't be an issue.

    I have to say that I really really like the Scirocco, but I just don't want to waste my time / money chasing a dream that will never happen.

    I'm getting very good velocities from these projectiles.

    I almost think that I should try another ladder test and see if they come right. The one thing that I have learnt since my first ladder test is that they should be loaded 50 thousandths of an inch off the lands, and my first test was 20 thousandths off.

    I have read that that makes a huge difference.

    I will say that the yawing is not extremely evident, but I do believe that the projectile is yawing at 100m.

    I would appreciate your thoughts.
     
  4. rscott5028

    rscott5028 Well-Known Member

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    My understanding is that once the bullet reaches gyroscopic stability, it stays that way pretty much until impact except for the transonic range which is really a separate issue. If it's unstable to begin with it probably won't get much less so. But, it's going to seem like it over long distances where you need better accuracy for a clean kill.

    Depending on your bullet, then your impact velocity should ideally be above 1800 fps for good terminal performance on game which is well above transonic. ...perhaps 2000fps or more for the Scirrocco?

    Gyroscopic stability is one factor affecting accuracy. You can sometimes find a load that matches the harmonics of your rifle and it will provide sufficient accuracy with a marginally stabilized bullet. You would just have to try it.

    I've wasted a lot of time trying to make slow twist rifles shoot well with longer bullets. Life is too short. I prefer to go by the calculator and focus on one that's a mathematical good fit.

    -- richard
     
  5. edge

    edge Well-Known Member

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    I may be wrong, but you may be on the very low edge of stability.

    Now that means if your crown is perfect, the gas exits perfectly square and the temperature is high and barometer is low then you should be fine :)

    On the other hand, a marginally stable bullet can't recover from yaw at the muzzle and in high pressure or low temperatures the bullet it may not ever be stable even under ideal conditions.

    edge.
     
  6. Outlaw6.0

    Outlaw6.0 Well-Known Member

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    ^^^This is how i've come to understand it as well, rotational velocity does not bleed off like translational velocity, your round should retain the majority of its rotational speed throughout the bullet flight.

    But here is the Caveat, if your are experiencing bullet yaw on paper it's no big deal, but you throw that same amount of yaw on a game animal you may be setting yourself up for a serious bullet failure. As bullets are designed to obviously penetrate in as straight a line as possible, gyroscopic stability is a must, if your bullet either hits the animal with a slight yaw or yaws while in the animal you will tend to see unpredictable penetration. Your shoulder shot may end up somewhere in the ham etc (think of it as a deflection). Also at work are the mechanical forces on the bullet itself, being designed to impact at the nose, they don't tend to hold up with all of that force being applied to side of the bullet.

    Just my .02.
     
  7. proload

    proload Well-Known Member

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    OK, well from all the posts that I've received, it seems that:

    1. The problem is mathematical. ie Trust the stability calculator more than the guy with one twitching eye in the gun shop.
    2. The stability calculator says that the stability is only 0.87 and therefore not stable.
    3. The projectile is unlikely to become more or less stable in flight.
    4. I can't change any practical parameters to the extent required to stabilize this bullet out of this barrel.
    5. Terminal performance would be unpredictable if the projectile was still yawing on impact.

    Therefore, sadly, my conclusion is to give up on the Sciroccos and try something shorter.

    Would you guys agree with my logic and my outcome here??
     
  8. Outlaw6.0

    Outlaw6.0 Well-Known Member

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    Yeah, I agree with your logic. I would probably switch projectiles as well.

    But hey, on the positive side, you get to play with something new!gun)
     
  9. rscott5028

    rscott5028 Well-Known Member

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    Don't throw the Sciroccos away. But, I think you should consider some other bullets.

    I never understood what you meant by "analyzing the holes in your targets?" Are the Sciroccos key-holing? Or, are the groups larger than you'd like to see?

    What sized groups are you getting and at what distance?

    What distances will you likely take shots when hunting?

    Stay away from the all-copper bullets. They are longer than comparable weight lead core bullets and hence need a faster twist.

    Some you might want to consider...

    speer
    100 Grand Slam SP 1.040" (BC=.351)
    100 Spitzer BTSP 1.050" (BC=.430)
    Sierra
    100 Pro-Hunter 1.041" (BC=.373)
    Nosler
    100 Partition 1.065" (BC=.384)
    Hornady
    87 V-Max 1.037" (BC=.400)
    100 Spire Point 1.062" (BC=.381)
    Barnes
    85 TSX 1.061" (BC=.333)

    -- richard
     
  10. Buffalobob

    Buffalobob Writers Guild

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    Test the bullet at 500 yards and see what happens.
     
  11. proload

    proload Well-Known Member

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    Hi Richard,

    Thanks for your points / comments.

    Minor ... we're entering into winter here, so it can rain at a moment's notice ... as a result, I laminated my targets as they tend to rip to shreds if they get wet and I can't analyze them properly when I get home. I shot 3 separate projectiles on the same day (Nosler BT 55gr, Nosler BT 70gr and Scirocco II 90gr). The holes in the targets from both sets of Noslers were as expected. Hole punched through the middle with an even rip/stretching of the laminating material and paper on all sides of the hole. ie it's an even circle. On the Scirocco's, I can see that the hole is not in the center of the rip, and the rip around the hole is slightly (about 1-2mm) exaggerated on one side for all holes. ie uneven circle. I shot about 25 of these projectiles on the day. So in short, Yes I think that there is minor key-holing.

    Inconsistent. I've done a ladder test, and found what I believed to be the sweet spot. Fired 5 rounds in that zone. 3 rounds group under 3/4" at 100m, 4th blows it out to 2", 5th blows it out to 4"

    I'm not opposed to trying again with a bigger jump to the lands as I think that that may have been a factor in finding a precise load, but my concern is whether it's worth doing if the projectile is unstable. What do you think.

    I chose the bullet for it's impact performance at high speed. In other words, I wanted to have a projectile that I could absolutely trust to perform the business at anything from 25m to 350m.

    Thanks for your comments in this regard. I do appreciate any input that you can offer.
     
  12. proload

    proload Well-Known Member

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    Hi Buffalobob,

    Unfortunately the longest range that I have access to is 200m.

    Cheers
     
  13. proload

    proload Well-Known Member

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    One more thought on bullet selection ...

    Yeah ... I'm not sold on solid copper ... but I do like a bullet that won't totally blow up on impact and that will retain most of it's weight by the time it's done.

    I wonder about standard grade projectiles at the sort of impact velocities that a 243 can produce (> 3000 fps). My view is that they tend to fragment at those sort of speeds and thats really bad for hunting.
     
  14. rscott5028

    rscott5028 Well-Known Member

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    You indicated the 4th and 5th shots opened up the Scirocco groups. Is it consistently the 4th and 5th shot as in the barrel may be heating up and not stress releived? Or, are you saying 3 out of 5 are in tight and 2 are pretty far out there?

    How well did the Noslers group?

    350M isn't extreme distance.

    You just need to pick something in the 95-100gr category for lead-based, or the 85gr TSX and then get out and shoot.

    Here's some load data...
    .243 Win + .243 AI Cartridge Guide

    Happy hunting,
    Richard