BALLISTIC COEFFICIENTS/Twist Rates

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by 300spanker, Aug 22, 2010.

  1. 300spanker

    300spanker Well-Known Member

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    I have been shooting 3 different 300 ultra's with different twist rates. 1in11, 1in10 and 1in9, and it seems that the faster twist gun is producing a better BC at 1000 yards. I am shooting 210 bergers at 3200 fps out of each gun. Is this possible. The 1in11 bc is around .630 the 1in10 .665 and 1in9 around .700. Is there any theory around this to support my situation, can a bullet stay off the ground a little better if spinning faster???. thanksgun)
     
  2. groper

    groper Well-Known Member

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    my theory is that you are getting some extra lift due to the faster spinning bullet and the stronger gyroscopic force holding it in 'nose high' attitude toward the end of its trajectory.
     

  3. Topshot

    Topshot Well-Known Member

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    What is the accuracy like at 1000 yards. Comparing the 1 : 11 twist to the 1 : 9 twist rifles?

    If the 1 : 9 is more accurate than the 1 : 11 twist, it might be an indication that the 1 : 11 twist is slightly unstable and this is showing up as an aparent lower B.C.
     
  4. J E Custom

    J E Custom Well-Known Member

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    +1

    Stability and drag can change trajectories so I'm not surprised that the BCs are different.

    A faster twist does not allways mean better accuracy.The heaver bullet needs to rotate at
    a optimum RPM and velocity will effect the required twist rate.

    If you tried to shoot the lightest bullet (A 110gr)in a 1 in 9 it would probably over rotate the bullet and accuracy would fall off at the velocities that the RUM could push it.

    If you only shoot the heaviest bullets in the RUM then a 1 in 9 will be ok, for all round bullet
    weights the 1 in 10 is ideal and for the lightest/fastest bullets the 1 in 11 is the better choice
    but the 1 in 11 somewhat limits you on bullet selection.

    So Each twist rate has it's place and bullet weight and velocity will normally determine
    the best twist for its use.

    J E CUSTOM
     
  5. LouBoyd

    LouBoyd Well-Known Member

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    Most ballistic computer programs are based on McCoy's McTrag computer program and do not account for any deviation in the spin axis and the direction of flight (ie yaw) of a bullet. The programs assume a bullet will fly nose into the wind. If a bullet is overspun it will fly "nose up as gravity tries to give it a roughly parabolic trajectory. I'm assuming you're measuring the effective BC from drop at the target and not by measuring the velocity at the target or time of flight.

    You many not be measuring effective BC at all, just yaw effects. Yaw will actually reduce the BC if the bullet is not flying nose into the air, but a nose up bullet will have some lift at least until it precesses by 90 degrees. If' that's doesn't happen until over halfway to the target it will appear as a higher BC based on drop alone. You could sort out which is happening by measuring the velocity at the target or measuring the time of flight accurately. I don't know of a convenient method other than two chronographs or three chronograph screens and a digital oscilloscope with a good time base.

    So which barrel gives the most accurate groups? That's much more important than which gives the least drop.
    Also it would be good to know what elevation (altitude) and air temperature you're shooting in. Air density does matter in determining bullet stability.
     
    Last edited: Aug 22, 2010
  6. 300spanker

    300spanker Well-Known Member

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    Thanks for all the info. I must be gettting a little lift or nose up on the 1in9 twist gun. Believe it or not all guns shoot very well at all distances up to 1000 yards. thanks again for the responses
     
  7. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    Spinning a bullet faster DOES NOT result in more of a 'nose-up' attitude, and no lift is generated in the vertical plane.

    Rather, the yaw of repose (in the horizontal plane) will be greater and the faster spinning bullet will have more spin drift (to the side).

    This behavior has been studied, modeled, and reported in this document:
    http://appliedballisticsllc.com/index_files/HATS_Report.pdf

    I'm curious to hear about your methods of determining BC for the 3 barrels.

    I'm not saying the results aren't real, I just don't think they're due to a nose high flight orientation. Induced drag from excessive pitching/yawing motion may be less for a faster twist barrel, but that would only affect the first ~100 yards of flight. Or, the faster twist barrels may be 'smoother', leaving the bullet surface more 'slippery' somehow.

    -Bryan
     
  8. groper

    groper Well-Known Member

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    I thought i was fact that overspun bullets didnt "nose over" when they start to fall from the middle of their trajectory so they then start to fly a little belly forward? i previously read that this usually caused a reduction in BC however, not an increase...
     
  9. 300spanker

    300spanker Well-Known Member

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    Bryan,
    Thanks for your response. The way i have determined the BC's is through exbal nightforce software by simply taking the original bc G1 plugging it in, using station pressure, bore height, velocity, sight in distance ect...... and then measuring hits at 1000 yards. I know i could then use trajectory validation, but i basically back into the software different bc's until they match my hits. then shoot the corrected bc's. The 1in9 bullet actually produces- fly's closer to a .730 bc on the 210, this may sound crazy but it does. Could the theory of finding that perfect spin or faster spin relate to the 6'8" 280 pound underhand fastpitch pitcher in northern az that actually spins the softball so fast his riser pitch strikes out the best of us. he spins the ball so much faster, i.e. backspins the ball so much better it actually rises. I know a bullet is not backspun, like a baseball, golf ball or softball, but if the rotation clockwise revolutions per second 3200fps 1in 9 twist versus others, could it create a force downward which would keep it vertical a little longer, as the example of our feared fastpitch riser-ball master. However, after 1000 yards the 1in9 twist bullet seems to slow down more that the other barrels. Could a faster twisting bullet shed velocity faster than a slower twisting bullet?? Can more twist create more air resistance because it is spinning faster thus velocity at some point is effected more? One last item, the faster twist bullet seems to open up on game much more violently, i.e. muledeer and antelope, it seems twist helps in that aspect. thanks, please reply. chris
     
  10. groper

    groper Well-Known Member

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    i guess what it comes down to is how sure are you of your zero, velocity and scope tracking? even small errors here will produce big changes in BC.

    For example, if your zero is 1 click off, then that translates to about 5 inches at 1000yds.

    If your scope tracking is out by 5% per click as my last scope was, thats approx another 10inches.

    and if your chrony or velocity guesstimate is out by 30fps, thats more again... so if you put all that into your ballistic calc and work back, you can get quite a large varience in BC...

    do you use the same scope on each rifle? are your velocities measured on the same chrony on the same day each time you test?
     
  11. Autorotate

    Autorotate Well-Known Member

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    Bryan

    Thanks for this post and your others as well. I, and many others appreciate your contribution here on LRH in helping the common man wrap their mind around some of the complex topics of exterior ballistics.
     
  12. 300spanker

    300spanker Well-Known Member

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    Guys,
    Thanks for the help, i will continue to try to fine tune it all. i may need to be more precise on all my calculations.
     
  13. J E Custom

    J E Custom Well-Known Member

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    +1 Its all ways good to hear from a bullet guy that knows what he is talking about.

    J E CUSTOM
     
  14. Kevin Thomas

    Kevin Thomas Well-Known Member

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    I don't know if this will answer any of the questions raised here, but I ran a series of BC firings that are reprinted in the Sierra Manual (3rd or 4th Edition, I think?) which addresses this exact issue. I shot a series of bullets through rifles of differing twists, in both .224" and .308" bore sizes. In the .224" as I recall I think I went with 1x7", 1x8", 1x9", 1x10", 1x12" and 1x14" using the 69 grain SMK. In the 30s, I went with 190s in a 1x8", 1x9" 1x10" 1x11" 1x12" and 1x14". Bill McDonald plotted out the results, and they were interesting. Bottom line here was, as Bryan had indicated, very little change in BC so long as the bullets were stable. As the SF approached 1.0, the scatter of the individual shots (the ES and SD of the BCs observed) opened up dramatically. As they dipped below an SF of 1.0, we continued to see round bullet holes (perhaps some excessive yaw, but no actual tumbling at that point) this scatter became general, with extremely reuced BC figures, as you'd expect. The point to this was to take both bullets from twists where they were "overstabilized" (no such thing, but it expresses the idea well) to a point where they were truly unstable, plotting the BC changes along the way. By way of full disclosure, the firings were done at 200-300 meters, which is all had available to me in the tunnel.

    Kevin Thomas
    Lapua USA