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Rate of twist

uni82uni82 Member Posts: 416 ✭✭
Best rate of twist for .024 Ruger? Looking at bullets of 45 gr and up, 1:9 or 1:12 twist?

Comments

  • uni82uni82 Member Posts: 416 ✭✭
    edited November -1
    What is the maximum bullet weight that I could shoot through my .223 with a 1 in 9" twist

    Thanks
  • uni82uni82 Member Posts: 416 ✭✭
    edited November -1
    How reliable are the rate of twist calculators? Are they accurate? I am unsure if the equations are correct but can someone verify?

    http://kwk.us/twist.html

    plug in the variables and it can calculate the rate of twist required for the stabilization.

    Thanks

    -JD
  • babunbabun Member Posts: 11,054 ✭✭✭
    edited November -1
    I don't buy it. The shape of the bullet has a lot to do with it also. There are the same weight bullets made in the same caliber that have too very different shapes. One can have the rifling touching area as long as a much heavier or as short as a lighter bullet.

    PS I just ran some numbers on that calculator, and it seems that it works only for a narrow range of specs. Didn't work for the .22lr, {low muzzle speed} or the opposite end like a
    8x56 Aust/Hung. {very short fat bullet}
  • nononsensenononsense Member Posts: 10,928 ✭✭✭✭
    edited November -1
    uni82,

    As with any generic calculator, this one has a reasonable degree of accuracy with regard to the twist rate, as long as the variables are entered accurately.

    Please be sure to read the entire explanation found below the calculator itself. Here is a short excerpt:

    "Ken Howell wrote about twist rate in the 07/1999 issue of Varmint Hunter magazine. He mentioned Greenhill's work began with cannons in 1879. Two quotes Howell took from the Textbook of Small Arms (published in 1929 in Britain) are notable. "In actual practice Greenhill's figure of 150 can be increased safely to 200 and still control the bullet." The classic equation is for solid, lead alloy bullets of specific gravity (SG) 10.9, and "when the density of the bullet is less than that of lead or the density of the resisting medium is greater than that of air, the spin should be increased as the square root of the ratio of the densities." As SG decreases, the gyroscopic inertia of the bullet decreases in proportion, and one needs to increase the spin to compensate.

    C.E. Harris, writing in the 08/1983 issue of the American Rifleman, noted Greenhill's formula was developed before spitzer boattail bullets and high velocity cartridges. He used a more modern analysis of gyroscopic stability, in which a factor of 1.4 is minimum and 1.7 is usually good. He found that the numbers given by Greenhill's original formula ranged from 1.5 to 2.0 for military type boattail bullets and were about 2.0 for bullets with either a flat base or short boattails.

    The basic twist rate calculator above uses Bowman's equation modified with the SG correction quoted by Howell. However, Don Miller has shown this older equation to not be accurate over the full range of bullet shapes and muzzle velocities. I plan to use Miller's more accurate twist estimator in this calculator."

    Be aware that the generalized answer will be in the ballpark but the answer does not increase in accuracy no matter how many numbers follow the decimal point.

    The second point is that the best accuracy is often achieved when the slowest twist rate need to stabilize the bullet is used. It's sometimes better to ask the members for experienced answers when you get outside the realm of generic twist rates.

    Best.
  • bambambambambambam Member Posts: 4,814 ✭✭✭
    edited November -1
    I don't think it's correct. I plugged in a 140gr 7mm Rem Mag bullet and it told me I needed 1-14" twist.
  • nononsensenononsense Member Posts: 10,928 ✭✭✭✭
    edited November -1
    bambambam,

    "I don't think it's correct."

    That's because the field of ballistic is not as simple as plug and play games. There are several thousand details and variables which figure into the accurate description of the proper twist rate for all of the bullets and their respective shapes and lengths. The twist rate calculator linked above is simply giving the recommended twist rate for that individual bullet (length) at your particular velocity. Change the bullet length and/or the bullet velocity and watch the rate change.

    From the discussion below the calculator:

    "However, Don Miller has shown this older equation to not be accurate over the full range of bullet shapes and muzzle velocities.".

    Your example is correct for the bullet you chose, traveling at the velocity you supplied. But remember that this applies only to that bullet at that velocity. It doesn't make the calculator wrong by any means since it's telling you that if you shot just that bullet at that velocity, you would only need a 1:14" twist rate in your barrel. Barrel manufacturers don't know which bullets you'll choose so they give the faster twist rate to handle a broader range of bullet lengths.

    My standard for 7mm barrels is a 1:8.7" twist rate in order for every shooter to be able to load the Berger 180 grain VLD bullets and keep them stable at necessary velocities. This automatically precludes the use of the 7mm-08 case with these bullets when fired from a shorter barrel. There just isn't enough velocity to make that particular bullet work. Other examples exist and are too numerous to name and discuss. These will fall under the often odious category of exceptions.

    I have a couple of thousand pages of notes and lectures I've written, along with probably 40 books on the subject but I still have questions. Most of the bullet/barrel makers have some information on their websites. There are also some well written books on the subject.

    Best.

    *edited to correct twist rate.
  • uni82uni82 Member Posts: 416 ✭✭
    edited November -1
    Awesome. That is what I was wondering. I have a 223 Remington 700 with a 1:12 twist. With this twist calculator, it calculated that I would need a 1:12.8 twist if I were to use a Nosler 60 grain bullet. I would never know if it runs good in my gun or not until I shoot it. That was my thing. I was just wondering if it could give me a starting point.

    Also I will be purchasing a browning BLR 358 Winchester shortly and that has a 1:12 twist instead of the 1:16 twist that the Ruger m77 has. I would rather have that and shoot 225 sierras or Nosler 225's. Not that I don't want to shoot 200's I just want mass like a 45 acp. slow and steady =)

    Thanks again fellas!!!!

    -JD
  • bambambambambambam Member Posts: 4,814 ✭✭✭
    edited November -1
    Nononsense,

    I think you're onto something there. I picked out a few diffrent bullets I have:162g(1-10.7"), 154g(1-11.4"), 140g(1-13.6"), 139g(1-12.1").

    I don't have anything heavier than 162g but it seems that the twist calculator is going with a lower twist as it gets longer(heavier).
  • Alan RushingAlan Rushing Member Posts: 9,002 ✭✭
    edited November -1
    quote:Originally posted by bambambam
    Nononsense,

    " I think you're onto something there. "
    [;)] No doubt about that! [;)]
  • Rocky RaabRocky Raab Member Posts: 14,137 ✭✭✭✭
    edited November -1
    Twist rate is actually based on two things: bullet LENGTH and velocity. The longer a bullet is, the faster you have to spin it to make it stable in flight. You can spin it faster EITHER by steepening the twist rate OR launching it faster - or a combination of both.

    In the days when all bullets were lead or lead core/jacketed, it was a reasonable convention to speak of heavier bullets needing a faster twist. Because we can't normally launch heavier bullets faster, there's a double whammy factor that requires an extra fast twist to achieve a quicker spin rate.

    These days, not all bullets are lead core/jacket, however. Given two bullets of the same weight, an all-copper one will likely be longer than a lead core/jacketed one, and will therefore require a faster spin rate. Bullets with other core materials such as the new lead-free designs also have to have different spin rate calculations due to their specific length-to-weight ratios. So the picture isn't as simple as it once was.

    But however murky the picture, it all boils down to achieving a minimum number of revolutions per minute (rpm) to make that bullet stable in flight. Any "stability formula" is actually calculating an rpm.

    Figure the launch rpm of a bullet (12/twist rate x 60 x velocity) and you'll be amazed at how fast it has to spin to be stable. Examples: a bullet at 3,000 fps from a 1-12" barrel spins at 180,000 rpm. A bullet at the same speed but launched from a 1-9" barrel spins at 240,000 rpm.
    I may be a bit crazy - but I didn't drive myself.
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