Long bullets require fast twist barrels?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Engineering101, Oct 5, 2013.

  1. Engineering101

    Engineering101 Well-Known Member

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    I have 8 rifles. Only one of them has what might be considered a fast twist barrel. It is a 260 Rem with a 1 in 8 twist. It shoots any bullet from 95 grains to 140 under 0.5 MOA. All my other rifles have “standard” twists. I have found shorter bullets that will shoot 0.5 MOA in most of them but none of them will do that with a long for caliber bullet. Seems like a pattern. The problem with standard length bullets is that the BCs suck.

    It occurred to me that maybe you need a twist that will “over stabilize” a bullet to get it to shoot really well though I have heard it stated that the faster a bullet spins, the more detrimental “imperfections” in the bullet will be to accuracy. I’m beginning to suspect that sentiment could be more folklore than fact however.

    If I’m barking up the wrong tree here it should be fairly obvious – there will be cases where guys have good shooting loads using long for caliber bullets in barrels that just meet the twist requirements for the bullet. Let me provide an example. The 338 cal 280 grain Barnes LRX is a long bullet and the box says it requires a 1 in 10 twist which is standard for that caliber. Is there anyone out there that has this bullet shooting 0.5 MOA or better in a 10 twist barrel? How about the 200 grain 30 cal LRX or 168 grain 7mm LRX?

    To be clear, I’m not talking about one good 3-shot group but rather multiple 4 or even 5-shot groups under 0.5 MOA. As I said one of my 8 rifles will do that with a long bullet and it has the fastest twist. Oddly my next best rifle is also my next fastest twist, a 1 in 9 twist chambered in 7mm WSM. It will stack 180 Bergers into 0.75 MOA. The rest of the rifles are pretty much 1 MOA rifles with the longer bullets though most have shot groups under 0.5 MOA with standard or short bullets.

    I'm looking for a couple of examples where long bullets shoot good in standard twist barrels and then I can then go worry about something else. Otherwise I may have to starting ordering some faster twist barrels. Thanks.

     
  2. Lefty7mmstw

    Lefty7mmstw Well-Known Member

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    Long bullets for a bore that are close to max. for your recommended twist are harder to get shooting well, but not impossible. My 9" twist ar in 5.56 nato consistantly puts five 75 to 77 grain hpbt's into a moa at 100 with peep sights, or slightly more if I'm cross eyed that day. My 7rem is shooting the 171 barnes match in a 9 and change rem barrel to 1" at 200. My 300 rum is eating the 225 hornady hpbt to .6 moa at 200yards. I do shoot more mid weight bullets than heavies though.
     
    Last edited: Oct 5, 2013

  3. MontanaRifleman

    MontanaRifleman Well-Known Member

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    What do you mean by "over stabilize"? The only thing I can think of to call over stabilized is when a bullet's RPM is so high that it doesn't nose over beyond the apex of its trajectory.

    Bullet manufacturers are usually conservative when it comes to recommending a twist rate. So if you are having accuracy problems with recommended twist rates, it's probably not the twist.

    Bench Rest competitors usually get the lowest twist rate bore they can stabilize a a particular bullet with so if your bullets are making round holes (stabilized) you should be able to shoot good groups with them provided the rifle is capable and it likes the load.

    Bergers twist calculator is a good one for most lead core bullets. Mono's being a lower specific gravity will need a tighter twist rate than same length lead core bullets.

    Bottom line is, if you are following manufacturer recommended twist rates you should be fine.
     
  4. trazman

    trazman Well-Known Member

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  5. MontanaRifleman

    MontanaRifleman Well-Known Member

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    That twist calc is a good calc for lead core VLD bullets but not for monometal because of the difference in material density.
     
  6. Mikecr

    Mikecr Well-Known Member

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    You're seeing that in a general sense it's better to have enough twist and greater, than not enough twist. A more detailed review reveals 'best' twist(Sg) for each bullet, load, and shooting condition.
    Best, or best balance of all pros/cons, can also be tied to particular use.

    Folklore spins out of misunderstandings, and actually end up pseudo facts with funny mutations.
    Bullets going to sleep is an example of a minor factor in external ballistics that has been hogwashed to explain all kinds of unrelated things.
    When someone declares that overspinning bullets is bad for accuracy, it really means overspinning is not quite as good as perfect spin. And of course at worst overspinning leads to jacket failures.
    But it's usually easier to see underspinning as a 'problem'.
    Also, predicting the perfect speed is not as easy (with some bullets) as running numbers through a rule of thumb calculation.. It still comes down to center of pressure to center of gravity length that disturbing moments leverage, and these can't always be measured with calipers. A resulting example here being that ideal Sg for flat base BR bullets(~1.3) is lower than ideal Sg for VLDs(~1.5), depending on the bias of your calculation method. With this, point blank BR shooters do go lower in twist than prudent for longer range BR shooters.

    But if you shoot short flat base bullets with a twist producing Sg of ~1.5, or even ~1.8, they will still shoot fine, as will long VLDs. Drop both down to calculated Sg of ~1.2 and it's likely both will shoot worse. With high muzzle pressures(barrel too short) and any less than a perfect crown, an Sg of ~1.1 may not be enough to prevent tumbling. And at ~1.0 bullets will immediately tumble.
    This is why it's easier to see underspin than overspin.

    I picked up a Cooper in 223rem, they come in 14tw. After buying, it dawned on me that 14tw is stupid, as the standard for this round is 12tw. I was right, there are few over the counter bullets that would work well out of 14tw.
    I did some calcs and tried 50Blitzkings with an Sg just under 1.2 here at sea level. Best I could get was ~1/2moa from them. Then I found 50Jayners(FB BR bullet) with a calculated Sg of 1.3, and my grouping dropped immediately to ~1/4moa. It was like flipping a switch.
    Where I hunt with these bullets(upper PA) their Sg is 1.44, and I have a very high kill percentage with them inside 600yds.
    So I'm left to wonder, if the Cooper were in 12tw, would I have reached this point accuracy-wise?
    I would have gone to 55-60gr bullets, better in the wind, but ya never know about accuracy. The gun would have bucked more, the powders would be slower, and my barrel of the same length would produce higher muzzle pressures.
    See, there is a lot to it that isn't in rules of thumb.
     
  7. trazman

    trazman Well-Known Member

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    I know that. How do you calculate the twist for monometal bullets?
     
  8. MontanaRifleman

    MontanaRifleman Well-Known Member

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  9. Mikecr

    Mikecr Well-Known Member

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    Actually, the Miller rule of thumb will work with solids, as uniform specific gravity is not needed given weight of a bullet.
    Border's calculator assumes weight of the bullet is not known, as it's a bullet design tool.
    Notice no bullet weight input field.

    Both will pass and fail tests. I believe Borders comes from a bullet design tool written by Bill Davis, using much of Robert McCoy's math.
    McCoy's approach represents more truth than rules of thumb can, but is difficult in use as many actual/difficult measurements must be taken for it.
    The miller rule of thumb is useful for typical bullets in use today, as it was with Greenhill a long time ago. And it fails tests with bullets on the edge of todays 'typical' scale. But it's way better than Greenhill.
     
  10. Engineering101

    Engineering101 Well-Known Member

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    Thanks to all of you who commented on my original post. There were several good points made. For one, it seems that a bit more twist than you need is better than a little less twist than you need. However, I’m still looking for an answer to my main question. I should have been more specific. So let me be and maybe that will help get to the crux of the situation.

    The bullets I’m having trouble with are the heaviest/longest Barnes LRXs available in several calibers, namely the 7mm, the 30 cal and the 338. Has anyone shot repeated 0.5 MOA groups with any of those LRX bullets and if so what bullet and what twist were you using? And if you don’t mine what was your chambering, load, jump, barrel length and velocity.
     
  11. RTK

    RTK Well-Known Member

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    I shoot the barnes 200lrx out of a 300wm, 26" rock creek 1:10 twist running a little over 2900fps.. Before the last 500+ rounds down the tube I was .040 off the lands, now I am about .070 off with no real difference in group size but a little slower confirmed by drops today.
    When I do my part I can keep them under .5moa at 100yrds.
    The last time out I was running a tad over .5 vertical at 750yrds shooting bi-pod prone with my fist and styrobead bag at the rear.
     
  12. Edd

    Edd Well-Known Member

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    What if the Sg is 2.8?
     
  13. RTK

    RTK Well-Known Member

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  14. roninflag

    roninflag Well-Known Member

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    Engineer- it is more a question of barrel quality. if you go to riflemansjournal, German shoots several different twist 30-06's. 10, 11, 12. generally the long bullets are a little more finicky, but my 7remmag shoots 168 bergers much better than the 140's even though it is shorter. also if you go to 6mmbr.com; the 6mmbr with an 8 twist ( fast ) shoots the light bullets great also; as well as the heavier ones.