rifling twist

Discussion in 'Technical Questions & Information' started by Maximilian II, Aug 4, 2009.

  1. Maximilian II

    Maximilian II New Member

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    Could an educated/experienced person please explain the relationship between the rate of twist of rifling to bullet performance please? i understand that bullet weight seems (I may be in error here) to respond/correspond to rate of twist. All applicable info is welcome!
    This is a new concept for me. Educate at will here!
  2. TranterUK

    TranterUK Guest

    I cant claim to be educated or very experienced, but as I understand it its to do with stability in flight. A given twist rate will balance better with some weights than others, resulting in a stable, consistent trajectory. Poor balance will result in less stability and bigger groups.

    One common calibre with a twist rate issue is our friend the 5.56mm, typically running 1 in 7", 1 in 9" or 1 in 12".

    I am sure more can be added....
    Last edited by a moderator: Aug 4, 2009
  3. 22WRF

    22WRF Well-Known Member

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    T = required twist IN CALIBERS
    T = 150 divided by the length of the bullet IN CALIBERS

    To figure out what twist you need, disregard weight, and just measure the length of the bullet in inches. Divide this measurement by the diameter of the bullet in inches. Next divide 150 by this new figure. Now convert the result back to into inches, and you'll have the result that tells you how many inches are allowed in which the bullet must make one complete revolution. (See page 556 in Hatcher's Notebook.)

    Example: A .30 caliber 220-grain bullet is 1.35 inches long, divided by .30 = 4.5 This bullet is 4.5 CALIBERS long.

    Next, required twist = 150/4.5, or 33.33 CALIBERS. 33.33 CALIBERS - 9.99" .

    So a .30 cal bullet, 1.35" long, of whatever weight can be stabilized by a 1 in 10" twist.
  4. TranterUK

    TranterUK Guest

    Thanks 22WRF, always something new to learn in this game.
  5. Maximilian II

    Maximilian II New Member

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    Aha! So it's the relationship between bullet length and diameter that dictates it. That's what I got, is that the simple description?
    Thanks!
  6. LDBennett

    LDBennett Well-Known Member

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    It is indeed the relationship of the bullet length to its diameter and has nothing at all to do with its weight. The bullet could be made of wood (much lighter than lead!) and the same twist rate would be required for the same diameter and length. This relationship is the Greenhill Formula. A google search will reveal a bunch of detailed explanations of the effect and the Greehill Formula.

    In a nut shell think of the bullet like a foot ball. As a kid when you threw a football without a spin, it wobbled in flight but give it the right spin and it traveled straight without any wobble. Its called stability of flight and the Greenhill Formula gives you the answer for various diameter and length bullets. It gives the minimum twist rate that stabilizes the bullet. But twist the bullet too fast and it may come apart in flight or wobble because the speed accentuates the internal imbalance of poorly made bullets (match bullets survive over stabilization much better than hunting bullets, for example). Stay with the popular choice bullet cartridge combos and most modern rifle have few if any stability of flight problems. If you wish to use heavier bullets run the Greenhill Formula on your choice to see if maybe you will have stability of flight problems (the bullet MAY shoot fine even though the twist is too slow... you have to test to find out).

    LDBennett
    Last edited: Aug 5, 2009
  7. Maximilian II

    Maximilian II New Member

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    Fantastic, this is what I was looking for.
    Thanks!
  8. armedandsafe

    armedandsafe Guest

    I got to meet Roy Weatherby back when he was still using Remington actions. He let me shoot a rifle he was playing with, Think 300 Weatherby necked down for .223 bullets of solid copper. He said each barrel would last about 800 rounds. The big problem was getting the rifling right, beause he had to build special rifiling tooling. Then he went into an explanation of what rifling meant and it boiled down to having to use solids because he couldn't rifle them slowly enough to keep standard bullets from blowing up at 4000+ fps.

    Pops
  9. colt

    colt New Member

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    Thanks, lots of very good info here.
  10. VegasTech702

    VegasTech702 New Member

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    you guys are leaving out a key element here, barrel length.
  11. Mr. Nameless

    Mr. Nameless New Member

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    Please explain, cause I'm lost on the subject.
  12. VegasTech702

    VegasTech702 New Member

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    I am by no means an expert on barrel specifics, but from my understanding the longer barrel you have means you can get away with a slower twist rate with a larger bullet. Barrel lengths has an impact on the stabilization of the bullet. If your barrel is too long and has a fast twist rate you have the possibility of over stabilizing the bullet which is not good either. It can slow the velocity down. In any case, I would recommend consulting with a qualified gunsmith that handles long range bolt guns or a barrel manufacturer for guidance when picking out the length, twist rate and contour of a new or replacement barrel. You want to tailor your barrel to the bullet you want to shoot.
  13. LDBennett

    LDBennett Well-Known Member

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    If you go to

    http://en.wikipedia.org/wiki/Rifling

    there is a brief description of the Greenhill formula.

    The accommodation this rule of thumb makes for velocity (the length of the barrel does indeed effect the bullet velocity) is in a different "C's " for below 2800 FPS and above 2800 FPS. Below the C = 150 and above 2800 FPS C = 180. That's not much of an accommodation but apparently the effect is not that great.

    Over stabilization is a misnomer as it is not mathematically possible to over stabilize a bullet. But if the construction is not perfect the extremely high rotational RPM of the bullet can make the bullet disintegrate. Better bullets (like Match bullets) are more tolerant of these high rotational rates. I found this out the hard way when I attempted to shoot 52 grain normal hunting bullets in my fast twist (1 in 8) AR varmint rifle and most never made it to the target. Slowing them down helped but the accuracy was not good. Changing to the same weight Match bullets was better yet but the best accuracy was achieved from this fast twist gun with heavy bullets (actually longer). I settled on 75 grain BTHP Hornady match bullets. I'm still developing the load but the group size is down to 0.75 inches at 100 yds (5 five shot groups averaged) and I expect it to go down some more. By the way these bullets feed from the magazine just fine.

    LDBennett
  14. Hammerslagger

    Hammerslagger New Member

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    Several members have made some excellent, very useful and valid contributions on this subject.

    I'll add my "two-cents worth" citing a couple of proverbs: "As you learn more; you learn there is much more to learn." And (from early 20th century Tokyo Jui-Jitsu) "One-hundred (tricks, throws, holds) easy to learn; one (principle) difficult to learn".

    Most "formula" answers for things like rifling twist rate are good starting points; nothing more. Actual performance is often influenced by other physical factors that are not addressed by the formulas. These include relatively small (or large) bullet diameters, atmosphere density and temperature, muzzle velocity, bullet design, and likely some other factors that do not come to mind at the moment.
    Competent information can be elusive, as the world is populated by self styled experts who are not such.

    Working from memory, I will use a partial history of the Eugene Stoner designed AR-15 rifle as an example; as was reported in several American Rifleman articles over a period of about 30 or more years.

    These rifles were originally traditional center fire .22 1:14 twist, which gave the 55 grain boat-tail military bullet (circa mid 1960's) a marginal stability factor of only .13. { So unstable that the bullet would become unstable (tumble) when it hit almost anything (soft point tissue destruction with a FMJ bullet), or even encountered sub freezing air temperatures. This is one reason the US Army rejected it during cold weather testing, saying that it was not suitable as a "battle rifle". Also it's marginal stability and weak bullet jacket was a factor that was used by its promoters who often demonstrated it against cinder block walls and watermelons as compared to the FMJ .30 cal. bullets of the day.}

    By the late 60's the military knew that at least a 1:12 twist was needed; but the jacket technology of the time would not stand up to a twist rate that fast. Come forward 20 to 30 years to the M-16 with a 1:7 twist to stabilize the 62 grain SS109 bullet so that it would perform more like a 147 -150 grain .30 cal. Spitzer bullet at 500 yards, and be able to penetrate a typical steel battle helmet.

    In recent years it (in .223 Rem., 5.56 Nato) has been adapted to shoot 80+ grain, boat-tail bullet for the national match "service rifle" course; shooting closer to the wind than the M-14 for "service rifle" competition. Yet most top competitors are using a significantly slower twist than 1:7 (like 1:9).

    The bottom line is that almost every bullet caliber, design/ velocity/ range/ and atmospheric conditions combination will have an optimum twist rate. In the real world one needs to find the best compromise for the conditions that will likely be encountered while doing the type of shooting that you intend to do.

    This kind of information is often available by consulting various publications that report on real world matches and accuracy testing. One who shoots under widely varied conditions (and/or using different loads) may need several rifles (or a rifle with several interchangeable barrels) to optimize his or her performance at a given time.
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