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BALLISTIC COEFFICIENTS/Twist Rates

 
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  #1  
Old 08-22-2010, 02:02 AM
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BALLISTIC COEFFICIENTS/Twist Rates

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???. thanks
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Old 08-22-2010, 06:10 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

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.
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Old 08-22-2010, 08:29 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

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.
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Old 08-22-2010, 09:27 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by Topshot View Post
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.

+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
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Old 08-22-2010, 11:35 AM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

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 by LouBoyd; 08-22-2010 at 11:46 AM.
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Old 08-22-2010, 04:32 PM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

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
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Old 08-22-2010, 08:30 PM
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Re: BALLISTIC COEFFICIENTS/Twist Rates

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/inde...ATS_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
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