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Thread: Twist rates: Is too slow a problem?

  1. #1

    Default Twist rates: Is too slow a problem?

    I'm looking at buying a new rifle. The model I'm eyeing has a 1:10" twist rate. Using the Greenhill Formula for my chosen bullet, the optimal twist rate is closer to 1:8.5". Should this concern me for shots <300 yards?

  2. #2
    New member George's Avatar
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    Default twist

    fremont, The Greenhill formula has been around a long time and can be used to get to "ball park" estimations for best twist rate. Bullet diameter, length, velocity, shape, density, weight, weight distribution, etc. all play a role in best twist. Assuming the rifle you're looking at is factory- they have probably used either a standard twist rate for the cartridge or a twist rate best for the most common bullet fired from that cartridge. Regardless of which twist rate a barrel has, there is usually only one bullet weight and style that will be best with all others being a compromise. Bottom line- no one best answer other than if you plan to use the "normal", mid-range or lighter bullet for your cartridge/rifle- the 1:10 twist will probably work fine. If for bullets in the heavier or longer than "standard" in that cartridge then they may not shoot as well as the slightly lighter, shorter ones. But, the Greenhill formula can only give an estimation for best twist... it is rarely exact. I know that barrel manufacturers usually say if unsure go with the faster twist. Downsides to the faster twist is that it tends to up pressure slightly and usually causes a little more fouling. Also, under normal circumstances, if the bullet is stabilized enough to group well to 100 yds, is more or less "standard" for the cartridge then you should see little difference out to 300. It's doubtful that if the rifle is "standard" twist for the cartridge and "normal" bullet there will be much difference between an 8.5" twist and the 10" twist out to 300. Additionally, the smaller the caliber the more critical it becomes for proper twist rates. For example: A slightly incorrect twist rate affects the stability of a .224 normal bullet much more than say a slightly incorrect twist rate affects the stability of a .358 normal bullet, particularly at the longer ranges. Sorry, no quick and dirty answer- just some info.

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    Default Twisting and Turning

    Fremont,

    It isn't likely that a manufacturer made a rifle with too slow a twist. I'm curious, what caliber you are contemplating that needs a twist rate of 8.5". Normally we're only concerned about this for long range shooting with precision. Marginally stable at low velocity calibers such as the 308 Win and the 223 Rem (wth 69 gr and heavier) are the most frequently considered. The 308 with the 173 match bullet needs a 11" twist for the 600 yard line. (and a 24" barrel to keep velocity up) The round will give 3" groups with 11" and 8" groups with 12" twist, at 600 yards. The standard 55 grain 223 is good in a 10" gun to about 500 yards. Beyond that accuracy falls off and when we go to the longer bullets a 9" is about all that is needed to 600 yards if barrel length (velocity) is 24".

    The Greenhill Formula: Twist=150*D (squared)/length of bullet in inches.

    Will usually yield good results. There are a number of web sites that have a plug in the numbers calculators for twist rate calculations. I think www.mountainmolds.com has one. Good shootin'

    Murphy
    Last edited by Murphy; 09-05-2006 at 08:48.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



  4. #4

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    260 Rem shooting bullets like Barnes or Oryx 156. For the latter, I get exactly 1:8.42" using Greenhill.

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    New member George's Avatar
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    Default long bullets

    Fremont,
    That's more to work with. I think, but may be wrong, those really long and heavier bullets like the Oryx 156 or the long by nature Barnes are better suited to such cartridges as the 6.5X55 or other 6.5s on longer cases. It may be too much of a good thing for the 260 Rem to handle anyway. It may become a "Catch 22" among seating depth, magazine length and throat length. I know that in my 260 the max cartridge length (magazine length) is about 2.8". Even with the two different 140 grain bullets I load, it requires very deep bullet seating with a lot of the bullet seated below the junction of the neck and shoulder- usually not the best idea. Aside from that possible glitch- I think you are correct in assuming the best twist for the 6.5mm 156 to be about 8-9". But, you may need to do a little more research on the rifle you are looking at. Two things to check would be magazine length and throat length.

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    Default More Twists...

    Fremont,

    I think the Steyr pro hunter I had in 260 Rem had a 1 in 9" twist. It would stabilize the 156 Norma Oryx. I think the Rem 700 series of rifles is also 1 in 9" twist, though I've never tried the oryx in one of them.

    I don't have any 156 Oryx on the bench now but my notes say that bullet was 1.240" in length and my calculations with Greenhill come out to 8.43" with the 150 constant. I have a 1 in 9" twist in my 6.5-06 and is truly an accurate 300 yard rifle with the even longer Sierra 160 grain HPBT match bullet. This is an Ackley version and I get about 2900 fps from that bullet so no problems with stability.

    I would agree with George, the 260 case is a bit small for the longer bullets and will perform best with the 140's. A 260 with 156 grainers would be like a 308 with 220's, not much room for powder.

    Good shootin'.

    Murphy
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



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