Twist rate vs velocity

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by 3006savage, May 19, 2009.

  1. 3006savage

    3006savage Well-Known Member

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    Anyone out there quantified or have a source that has documented what the velocity differences are in different twist barrels? Is there a rule of thumb?
     
  2. Mikecr

    Mikecr Well-Known Member

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    I don't see how this could be reduced to a rule with any accuracy.
    Afterall, there are so many other variables affecting velocity along with twist..

    One cal and cartridge might react to twist differences to a greater/lessor degree than the same cal in another cartridge(or even another barrel of same twist). Different powders and system pressure curves..

    So it wouldn't just be a matter of adding/subtracting energy converted to torque.
    I don't even know that you could predict whether velocity would go up, or down, when comparing 6ppc -vs- 6-06 for different twists. The powders and pressure curves(and usually barrel lengths) are so different.
     

  3. BountyHunter

    BountyHunter Writers Guild

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    Just too many variables for anything closely resembling a rule IMO.

    regular vs canted rifling vs gain twists
    freebore and lead variances
    Bore dimensions (tight vs standard bore)
    diameter of bullets (fat vs thin for caliber) Hornadys are known for being "fat"
    primer variances effect both MV, SD/ES and pressures greatly

    BH
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    Mike and BH are spot one here. Too many other variables come into play.

    The accepted theory is that all else being equal, a tighter twist barrel will produce slightly higher pressure than a slower twist barrel. This is straining gnats though.
     
  5. Katbird

    Katbird Member

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    Twist is all about stabilizing bullets. The goal is to have the least amount of twist that will stabilize the bullet you use. Twist rates have to do with bullet length, not weight, although they are generally related. However, one can get by with a slower twist if the velocity is increased, since the RPM of the projectile is what does the stabilizing.
     
  6. Mikecr

    Mikecr Well-Known Member

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    I don't know if Katbird is addressing your question, but his 'twist understanding dump' should really be corrected/enhanced..


    My goal is to FULLY stabilize bullets, as soon as possible.


    This is only a general rule, in a very limited sense. I'm sure many have screwed themselves with this notion.
    RPMs do go up with velocity, and stability usually goes up with velocity(a very tiny amount), but not necessarily.
    It depends on where the velocity puts you on that particular bullet's drag curve.

    Reason being, stability is tied to displacement per turn. It's the upsetting quantity for each turn to overcome. With this, bullet twist requirements are expressed as such (like 8"per 1 turn) based on 8" of standard atmosphere displaced, and the drag coefficient of that bullet, at chosen velocity, per 1 turn.

    There are other physical attributes of bullets affecting stability, but TIME never comes into play.
    Stability doesn't really follow turns per time(RPMs). If it did, bullet stability requirements would be expressed in RPMs,,, and none are.
    They never will ever be, because a time based definition of one condition would fail many tests elsewhere.
    -For instance, a bullet barely stable at 80deg, can whack paper sideways at 40deg. Same velocity, same RPMs, but different displacement(denser atmosphere).
    -In contrast, an unstable bullet at 1,500fps MV can punch round holes at 2,500fps. A huge improvement, but the bullet is far from fully stable, and would likely perform poorly. It can be the 40deg mishap above in waiting.
    So velocity helped, yet it would take an impossible 15,000fps MV to fully stabilize this bullet, with this twist and condition.
    So why such a gigantic change in RPMs to improve stability(from 171Krpm to 1029Krpm)? It's because RPMs never mattered..
    I used 10.5:1 twist for this test and whether 1,500fps or 15,000 the displacement was still 10.5" per turn. The difference, all along, came down to atmosphere and the bullet's drag curve.
    I could shoot it FULLY stable, at 2500fps, with use at 90deg and 6000' above sea level. Or I could choose a 9.5tw barrel from the beginning for use at 90deg at sea level. Or I could choose a 9tw barrel and always be fully stable.

    I'm no expert, so you might run tests for yourself at JBMs.
     
  7. 3006savage

    3006savage Well-Known Member

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    It does not seem based on all the responses like velocity is a consideration if one is on the fence trying to decide between two twist rates.

    I would think if the same reamer is used on several identical barrels (minus the twist rates) many of the variables at play could be eliminated/minimized. These barrels could be tested with several different loads made with indentical loading procedures. Pressures and velocities could be recorded to determine their relative performance.

    I wondered also if one would find that on average max charges may increase with the slower twist rates while pressures remain equal to the faster twist barrel. It would be fun to test but I sure would not want to foot the bill for that much testing.

    My thoughts on twist and velocity were not really the torque losses on the bullet but the additional bearing surface created by faster twist rates and possibly more importantly the angle of the rifing impeding the forward motion of the bullet. In theory At some point the rifling would stop the bullet completly since the friction created by the rifling would block any movement kind of like a threaded bolt.

    I am not trying to be arguementative and will accept the experiece of others that velocity is not affected by twist rate. I just wanted to put my thoughts out there I guess.
     
    Last edited: May 21, 2009
  8. BountyHunter

    BountyHunter Writers Guild

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    Now that is not the question you asked originally. You asked if there is a rule of thumb.

    But to answer the last question, look at the 240 SMK, Sierra says you should have a 9 twist. Why? It was origianlly designed for 2600fps for NRA LR comp in the 30-338 and 300 Win which were the two LR comp calibers then.

    Yet today we normally run the 240 in 10 twists at 2800-2900 with larger cases such as 300 Wthby and 300 ack imp and smokin groups. So is it twist, stability or what? I just know that you do not have to use a 9 twist for sure. I also know it does effect MV and pressure from what I have seen using a slower twist.

    I have found that slower twists allow more MV with less pressure and absolutely no loss of accuracy out to 1000 for sure, but again very barrel dependent.

    As for all the subtle questions you asked, the answers would only apply to that one barrel, chamber, twist throat combo and what is the significant impact. you could do all that testing but what does it get you that has real applications.

    Barrels are like twin women, they may look alike and have same parents, but boy are they ever different.

    To me that amount of endless testing is kind of like "pole vaulting over mouse turds". You can do it but so what?

    BH
     
  9. TOM H

    TOM H Well-Known Member

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    I think the best source of information on bullet/twist might be from who's barrel your going to use. I own afew rifles same caliber but use different barrels in using same loads I don't see a big velocity spread. I always work up my load first then chronograph it fun to compare and I limited how many bullets I'm going to try. 30-06 I use a Lilja 1/11 25" long barrel and Bartlein 1/11 5R barrel for other,270 use a Hart 1/10 other Lilja 1/10 and the 30-338mags use a 1/10 other 1/12 twist barrel. I've done the same thing with some of my varmit rifles.

    All the barrels are alittle different but not that much maybe I just got lucky.
     
  10. Michael Eichele

    Michael Eichele Well-Known Member

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    I wouldnt say you got lucky. As long as youre using the right bullet for the twist, all is well. There also isnt alot of practical difference between a 10x and an 11x barrel.

    A big, noticable difference would be using a 155 VLD or AMAX type bullet in a 9 or 10x barrel versus a 14x barrel. Now the difference here in accuracy AND velocity will be very noticable.
     
    Last edited: May 22, 2009
  11. TOM H

    TOM H Well-Known Member

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    I've been lucky in that I shot the HBR matches with a 308 using a 168gr Bruno FB bullets when I quit was using a 30x44 with 1/18 twist shooting BIB 112/118gr bullets.


    I shot a Kreiger 1/10 twist in 30-06 and that barrel was alittle faster with the heavier bullets than the 1/11 twist barrels using the same loads. I've got a 1/13 twist Ratchet rifled 4 groove barrel from Shilen chamber in 30-06 that my 150/165gr bullet rifle.

    Are you using a 1/14 twist with the bullets you mention?