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What is known about helical flight paths in bullets?

  1. Oct 31, 2016 #1
    I have a physics degree, I'm now retired and have taken up long range shooting as a hobby. When shooting at 600 yards, my spotters have noticed the contrail of my bullet follows a helical trajectory near the target. I shoot a 243 (6mm), 105 grain target bullet made by Berger at a muzzle velocity of about 3200 fps. When I spot for other good shooters (shooting a minute of arc or better at 600 yards), I notice their contrails show a straight path to the target, the only curvature of the path is an arc due to bullet drop.

    There are all kinds of shooting experts whose expertise seems to be more legendary according to the length of their drawl. They have all kinds of seat of the pants theories, but I was wondering if anyone has ever studied the physics and figured out why this happens and how to correct it. I load my own target ammo, so it's relatively easy to make adjustments.
     
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  3. Oct 31, 2016 #2

    berkeman

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    Welcome to the PF. :smile:

    So you've already discounted cross-winds as a cause?
     
  4. Oct 31, 2016 #3

    Bystander

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    Google "tractability." It/they do follow helical paths.
     
  5. Oct 31, 2016 #4

    Bystander

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    Amend this to "tractability"+"ballistics." Sorry 'bout that.
     
  6. Oct 31, 2016 #5

    Filip Larsen

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    I am no expert in external ballistics, but if the helical (possibly also called spiral or "squiggly") flight is observed to be in the initial segment of the trajectory I would guess that bullet instability might explain this.

    For a good allround introduction of the various effects I recommend Applied Ballistics for Long Range Shooting by Bryan Litz [1]. The 2nd edition (which I have) has a chapter on bullet stability which describe the dynamical mechanism behind such "squiggly" initial trajectory.

    [1] http://appliedballisticsllc.com/products/books/
     
  7. Oct 31, 2016 #6
    To Berkman: Cross winds were not a factor on the night we observed this. Our range has wind flags and they were barely moving. Also, I observed two other shooters the same night with straight paths (and better scores).

    I do not understand the tractability comment from Bystander. Are you saying all bullets do this? That's not what I observed, and several experienced shooters commented they hadn't seen it before. All bullets do precess about the flight axis and helical flight has been observed in bullet flights. It's hard to measure the diameter of the precession, but using the rings on the target thru a high quality spotting scope at 30X, I would say the diameter of the helix was about 2 to 3 minutes of arc.

    I want to know if there is some change in bullet selection or powder charge (reducing muzzle velocity) or something else that would limit the precession to less than 2pi inches at 600 yards (1 minute of arc).

    Thank you both for your interest and your help in clarifying this problem.
     
  8. Oct 31, 2016 #7

    berkeman

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    Do you use hand-loaded ammo? If so, could the bullets be slightly offset or have some other systematic defect that would cause the wobble that @Filip Larsen mentions?

    Have you tried swapping rifles/ammo with another shooter who also shoots .243 to see if the wobble follows the ammo or your rifle? If it follows your rifle, could it be a ding at the muzzle that is causing the wobble?
     
  9. Oct 31, 2016 #8

    Bystander

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    Just how "precise" are you being with your loading? Custom dies? Tolerances on powder weights? Free-floating barrel/bull barrel? Wood stock/composite?
     
  10. Oct 31, 2016 #9
    I hand load using a Lyman Gen 2 powder scale that is accurate to .1 grain. I use Lapua brass from the same manufacturing lot and all weighs within 1 grain (I sampled one in ten cases out of 400 cases). I check and adjust bullet concentricity to within .001" using a Hornady gauge. The gun has a Kreiger bull barrel, 28" in length with a 1:8 twist and 5R rifling, free floated on a composite stock. The barrel is pristine with only about 500 rounds through it. There are no observable flaws near the muzzle. The bullets are Berger 105 gr target bullets which come highly recommended by other shooters: G7 BC = 0.265; G1 BC = 0.517; Optimal Twist = 1:8". The bullets are seated to .01" to the lands in the barrel to minimize 'jump' and any axis offsets it may cause.

    It would be interesting to try these in another gun, but because I seat to the lands in my rifle, they may not fit or may perform poorly in another rifle. I could try the same load using SAMMI specs, but that introduces another source of error.

    I also have Litz's book and I did read the section on bullet stability. My observation was near the target, six hundred yards away. The wobble that Litz and others talk about is at much shorter range, maybe 100 yards or less and dissipates as the bullet travels further and has more time to stabilize. That wobble is inferred from poor groups at short range and better groups at longer range, since you can't see much of a contrail at 100yds.

    Thanks again for your interest.
     
  11. Oct 31, 2016 #10

    Bystander

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    Interesting.... Give me a little time to "gnaw" on this.
     
  12. Nov 1, 2016 #11

    CWatters

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    Not my field but google suggests there is an effect called "Epicyclic Swerve" that causes a helical flight path. For some bullets it appears the effect increases as they slow down and for others it reduces as they slow down.
     
  13. Nov 1, 2016 #12
    This effect could be caused by asymmetric or damaged rifling in the barrel of your weapon.
     
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