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BALLISTIC COEFFICIENTS/Twist Rates

 
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  #8  
Old 08-22-2010, 10:53 PM
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Posts: 327
Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by bsl135 View Post
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
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...
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  #9  
Old 08-23-2010, 12:59 AM
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Posts: 51
Re: BALLISTIC COEFFICIENTS/Twist Rates

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
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  #10  
Old 08-23-2010, 01:59 AM
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Join Date: Sep 2008
Posts: 327
Re: BALLISTIC COEFFICIENTS/Twist Rates

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?
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  #11  
Old 08-23-2010, 02:24 AM
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Join Date: Apr 2007
Location: ALASKA
Posts: 504
Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by bsl135 View Post
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
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.
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  #12  
Old 08-23-2010, 12:19 PM
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Join Date: Aug 2008
Posts: 51
Re: BALLISTIC COEFFICIENTS/Twist Rates

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.
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  #13  
Old 08-23-2010, 07:45 PM
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Join Date: Jul 2004
Location: Texas
Posts: 5,439
Re: BALLISTIC COEFFICIENTS/Twist Rates

Quote:
Originally Posted by Autorotate View Post
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.
+1 Its all ways good to hear from a bullet guy that knows what he is talking about.

J E CUSTOM
__________________
"PRESS ON"
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  #14  
Old 08-24-2010, 01:09 PM
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Join Date: Feb 2009
Location: Sedalia, MO
Posts: 1,253
Re: BALLISTIC COEFFICIENTS/Twist Rates

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
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