Rate of Twist and Accuracy

Discussion in 'Big Stuff -- 6.5mm, 7mm, 30 Cal' started by bsekf, Oct 10, 2018.

  1. bsekf

    bsekf

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    I am contemplating build a 7X47L to shoot 140-150 gr. bullets. Berger recommends 1 in 11 or 1 in 12. Barnes tested a 7-08 with a 1 in 9.5. Couldn't find twist rate info for Seirra, Nosler or Hornady.

    How do you decide on the best twist for accuracy with a given weight bullet. Is there a formula that involves BC? Or is it based on bearing surface and diameter? Will a 150 Berger require the same twist as a 150 Nosler?

    I was thinking 1 in 9 or even 1 in 8, but now think that is too fast and I would be better off with 1 in 10 or 11. I understand the heavier the bullet the faster the twist to stabilize it. But, there must be a rule of thumb or a formula.

    Bill
     
  2. kyreloader

    kyreloader Gold $$ Contributor

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  3. Papa Charlie

    Papa Charlie Silver $$ Contributor

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    Agreed go to the link above. You can play with all kinds of combinations. I just did some what ifs based on what you are thinking and it seems to align to the 1:8 twist. But you need to play with all the numbers and see for yourself.
    Remember to click on the Calculate Stability button each time you change a parameter.
    Have fun!
     
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  4. Ggmac

    Ggmac

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    Remember it's not weight it's the length ! Copper mono bullets are longer compared to lead core and VLD bullets are also longer due to ogive design
     
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  5. damoncali

    damoncali Gold $$ Contributor

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    A slower twist will give you better accuracy. For the best, you usually want to shoot for an Sg of 1.3-1.5 with your bullet in your conditions. If you just use sea level to calculate it it will be conservative (faster than necessary).

    Most shooters overspin their bullets these days. There are some good reasons to that ( you maitain the flexibility to shoot heavier bullets), and bad reasons (they think it helps BC, but there's not much reason to think it helps beyond 1.5 or so, and even then it only helps a tiny bit over 1.3). But you always degrade accuracy with faster twist, in proportion to the change in twist.

    You don't want to go below 1.3 or so, because you run the risk of losing stability.

    If you can get away with a 12, and you know you don't need to shoot something that needs a 9, stick with the 12.
     
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  6. DirtySteve

    DirtySteve Gold $$ Contributor

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    If you are wanting to shoot past the trans sonic zone you should go with a fast for caliber twist. The added stability helps keep the the bullet stable through the transition zone. Litz wrote about it in one of his books.
     
  7. mgunderson

    mgunderson sling shooter Gold $$ Contributor

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    My impression is the various disciplines arrive at a different twist rate based on placing different importance to the optimization criteria.

    Benchrest and fclass for example value precision, bc, recoil, and probability of bullet blow up differently. This is not a complete list but will generate a different optimum solution.
     
  8. JRS

    JRS Gold $$ Contributor

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    1.5-1.9 Sg.
     
  9. Evan

    Evan

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    I've seen "slow twist is more accurate" repeated a lot, but I've not experienced or seen any proof of it. Conversely, the BC drop from low stability is scientifically proven and measurable by anyone who tries to shoot a bullet with marginal stability.

    I'll qualify all of this with: I've never competitively shot aggs in benchrest and have not religiously tracked my groups long enough to have statistical data, so I wouldn't have a way to detect or quantify a slow vs fast twist effect that I'd estimate to fall somewhere sub 0.1 moa.
     
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  10. damoncali

    damoncali Gold $$ Contributor

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    The loss of accuracy from increasing twist is provable/proven scientifically as well, and it is more dramatic than the change in BC. It's proportional to the change in twist. Guys that are struggling with a 9 twist and getting 3/4 minute groups could go to an 11 (if their bullet allows) and pretty much instantly be getting 1/2 accuracy or close to it.

    You don't need a benchrest rig to see it clearly, but BR illustrates the difference pretty clearly. A .30BR is typically barreled in the neighborhood of an 18 twist, which is a little more troublesome to come by than a 12 twist. The reason for that is the increased precision. It doesn't make sense to shoot a 30 BR beyond 300 yards or so, so it's far more accurate to go with the shortest practical bullets and the slowest twist that will give adequate stability. In this case something in the 115ish grain range and a twist in the 18-19 range, depending on the exact bullet. This combo hammers at short range.

    It's all related to aerodynamic jump (among other things, but AJ is the biggest factor) - the bullets sensitivity to tipping as it leaves the muzzle. You can calculate this pretty precisely. You can also easily test it indirectly by varying the weight of the bullets by using shorter and longer lead cores in the same jacket. Or at least you can test the validity of the equations governing it. I've done this and the math is right, and twist rate is baked into the math. There is also quite a bit of engineering literature out there from the BRL/military on the topic.

    (Transonic effects aside, which get more complicated and uncertain - but 99.9+% of round fired never see the transonic region).
     
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