twist rate

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by SB49, Feb 19, 2012.

  1. SB49

    SB49 New Member

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    if a 70 grain ballistic tip will shoot just fine from a 6 ppc with a 1 in 14 twist at 3200 fps then why wont it work if i change the cartridge case to a 243.what does the difference in cartridge have to do with it?
     
  2. Mikecr

    Mikecr Well-Known Member

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    If you're talking about a Nosler 70gr BT, it's marginally stable in 12tw, and only 'possibly' stable in 14tw -at about 100degs and high altitudes.
    Then you have 2 different barrels, and with different bullet release(higher muzzle pressures with a 243).
     

  3. TOM H

    TOM H Well-Known Member

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    I've never shot a 70gr bullet of any kind in my 243/243AI both have 1/10 twist barrel and I may be old school if I want to shoot 68/70gr bullets I've got couple varmint rifles chamber for the 6ppc both have 1/14 twist barrels. I use one of the 6ppc around timberline shooting rock chucks pretty nice rifle.

    I wish I could help you more.
     
  4. SB49

    SB49 New Member

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    like i said the 70 gr shoots real good in my ppc varmit rifle with a shilen 6 groove 14 twist barrel, but i'm getting tired buying these expensive casings and then going through all the bullcrap of neck turning and fireforming.. it is to time consuming.
    i just want to re-chamber it to a 243 with a 265 neck and eliminate all the ppc case hassel.... but screw the twist rate theory, with the same barrel,same bullet, and load it a little light the 243 would shoot pretty much the same speed as the ppc, but all i hear is no no no the 243 needs a different twist rate... and you know what???? nobody can tell me why. and why would it all your changing is the casing, what does that have to do with the bullet stablelizing, its the same barrel,same bullet,same speed, how does the bullet know you put a slightly bigger casing behind it. its something like the 223 verses the 222 one is slow twist one is a faster twist... understandable one shoots a light bullet well the other a heavy bullet well,... but in my case its the same bullet so why does it require a different twist???????????
     
  5. Bart B

    Bart B Well-Known Member

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    Every bullet needs to be spun at a narrow rpm range to stabilize it in flight all the way to the target; even targets a long ways away. It doesn't matter what their muzzle velocity is as long as they're spinning at the best rpm to make them accurate. A given bullet shot through a 1:10 twist barrel that's super accurate can be shot about 30% faster in a 1:13 twist and it'll shoot just as accurate.

    Which is which it's wise to adjust the twist rate to the speed the bullet leaves at to stabilize it just right. Spin rate in rpm equals 720 multiplied by muzzle velocity divided by 12. A bullet leaving at 3000 fps from a 1:12 twist barrel's spinning at 180,000 rpm.
     
  6. TOM H

    TOM H Well-Known Member

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    I guess I may of misunderstood what you were asking didn't know you were going to rechamber that barrel. Your barrel will handle the 70 gr bullet in a 243 case.

    Well good luck. sorry about my first post
     
  7. Mikecr

    Mikecr Well-Known Member

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    What specific bullet are you talking about?
    Are you sure the Shilen is actually 14tw(measured)?
     
  8. edge

    edge Well-Known Member

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    Have you shot this bullet in all conditions?

    A bullet that is marginally stable at 50 degrees my not be stable when the temps drop into the teens, or when the pressure is very high.

    Most twist calculators take into account a bullet with some yaw and under the worst conditions.
    If you know that you will not be shooting in those conditions or that the temperature will not drop much lower than when you benchmarked this load.

    edge.
     
  9. SB49

    SB49 New Member

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    well it better be when i ordered it from shilen i ask for a 6mm 6 groove 1 in 14 right hand twist barrel. and i feel confident that is what they sent me.. and it was stamped on the end of the barrel that way.
     
  10. Mikecr

    Mikecr Well-Known Member

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    They're not trick questions:
    What is the specific bullet?
    Does your Shilen actually MEASURE 14tw?
     
  11. rscott5028

    rscott5028 Well-Known Member

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    Since you're having difficulty accepting the advice/explanations provided, you should simply give it a try. Due to the case sizes, it shouldn't be too difficult/expensive to re-chamber (and open the bolt face, which you'd have to do even with a new barrel).

    If it doesn't perform to your expectations, then buy a barrel with optimal twist for your application.

    A twist that's too slow doesn't mean that groups jump from .25" to 3". They're just not likely to be as tight as possible and are more apt to induce unexplained fliers. The latter is the biggest frustration I had when this has come up before and it's why I don't mess with it if the math doesn't add up.

    I'd love to see your before and after targets.

    Good luck,
    richard