Twist rates for elr?

Discussion in 'Extreme Long Range Hunting & Shooting (ELR)' started by RockyMtnMT, Feb 14, 2019.


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  1. yldouright

    yldouright Active Member

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    Keep in mind that SG and dynamic stability compete. Once the bullet goes 30° off the firing line, dynamic stability trumps SG. This is particularly critical once you go transonic with long bullets. The pressure leverage will flip and tumble the bullet. Gyroscopic stability is like salt in your food, too much makes it too hard to correct.
     
  2. NEMTHunter

    NEMTHunter Well-Known Member

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    Could you explain please? The faster the bullet spins the less trouble it has going from super sonic to subsonic. IE through the trans sonic range.
     
  3. yldouright

    yldouright Active Member

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    @NEMTHunter
    A very high, or even a comparatively high SG will keep the bullet pointing in the direction of the line of fire but when the bullet begins dropping, an angle divergence occurs and the bullet starts tilting belly forward. In this state, wind effect gets exaggerated and penetration suffers. When approaching transonic, this long bullet starts tumbling. A more moderate spin allows the bullet to enter the transonic state less abruptly because the center of pressure has less torque to overcome. Chey-Tac figured this out for us years ago and that's why their system can do what others can't. If your terminal velocity is likely be above 1250ft/s, spin away to your hearts content but if it's likely to be less, I wouldn't recommend a high SG.
     
    Last edited: Mar 14, 2019
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  4. jasent

    jasent Well-Known Member

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    That’s the opposite of my experience.
     
  5. NEMTHunter

    NEMTHunter Well-Known Member

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    Thats interesting. The reason I asked was From what I have heard and see A under spun bullet will tumble long before a "over" spun bullet will. At least that been my understanding. From what the ELR guys have said. Its better to have more twist then right on the edge. Specifically when going through the tran-sonic range.

    I can easy test the over spun farther since my "big" 30 cal is a 7.5 and The 230's are 3300 plus fps at muzzle.

    Interesting topic for sure. Would like tho see how that plays out. The thing with the chey-tac are there system is not the one winning the ELR matches.

    Thanks for answering. Keep the Data coming.
     
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  6. osoh

    osoh Well-Known Member

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    As far as the ELR matches and whats winning, trends and why. The 416s have become popular only because of seeing trace better with the larger bullets, seeing splash better in the dirt, rocks and sand at the 2000 plus yard range over the 375s

    I will be pushing a 505 grain 416 in the 3250 fps range out of a 39” 8 twist this year we will see what happens. Last year I was pushing a 400 grain 375 at 3361 fps out of a 35” 8 twist as well.

    Osoh
     
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  7. slv hunter

    slv hunter Well-Known Member

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    Good stuff. So what's the " perfect" SG.
     
  8. osoh

    osoh Well-Known Member

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    When it comes to long solids and ELR I still refer back to the 20 cal per turn formula and this will get you to a optimum choice.
    20 x .375 = 7.5 twist. As stated we ran 8 twist
    20 x .416 = 8.32 twist and yes I will be pushing the envelope at again a 8 twist on the 416

    Cheers
    Osoh
     
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  9. yldouright

    yldouright Active Member

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    This dynamic is dependent on the bullet shape. In long bullets with a high pressure lever, the effect I describe above is undeniable. When the bullet shape is more conservative, that lever doesn't need to work as hard because the bullet center of gravity isn't as skewed rearward. In those cases, more spin is more stable but as the bullet front gets narrower and longer, you'll need to drop your spin.

    There are reasons beside the bullet performance which come into play there and here are some:
    1. Experience with the load - Chey-Tac systems are expensive both to run and shoot so their limited adoption, limits their success.
    2. ELR rules - the Chey-Tac systems really shine above the ELR rule limit.
    3. Budget constraints - those willing to shell out for the Chey-Tac might be tempted to skimp on the other critical components for success like the optics, mounting, cleaning chemicals, etc.
    4. Terminal ballistics - the Chey-Tac will hit with the bullet point forward, which is a very small point of contact. In the case where a Chey-Tac round misses the steel by 3mm, a more conventional round tumbling into that zone will make a resounding hit.
    5. Shooters have adopted the Chey-Tac philosophy and lowered their spin in order to have their bullets track better and avoid the effects I described above.

    There isn't one, it's all situation specific and an art form. Personally, I enjoy working the numbers to make my choice.
     
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  10. jasent

    jasent Well-Known Member

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    I wonder why my long heavy for caliber vld style bullets that are “over” stabilized transition so well?
     
  11. yldouright

    yldouright Active Member

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    @jasent
    I'll assume you know how well they're transitioning because you're reading the result after the hit. If you're hitting above transonic, the issue is not as critical. If you're not, then I don't see how that's possible.
     
  12. jasent

    jasent Well-Known Member

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    Yes bullets hit nose first past transonic, past subsonic with repeatable results.
     
  13. jasent

    jasent Well-Known Member

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    Most rifle bullets eventually achieve hyper-stable flight if they fly far enough. Gyroscopic stability Sg increases throughout the flight. By achieving it right out of the muzzle, however, the rifleman is minimizing yaw-drag on his bullets during the critical first several coning cycles of flight where the bullet's coning "angle-of-attack" is usually largest and where the "potential drag force" (q*S) is always its very highest.”

    Jim Boatright
     
  14. yldouright

    yldouright Active Member

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    @jasent
    Are you calculating your terminal speed or reading it with a chrono? If you're estimating by calculation, show the calc/table spec. I don't want to insist but one of us is missing critical information and I want to be sure it isn't me, thanks for the marketing quote :)