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Twist Rate???

 
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  #15  
Old 04-02-2009, 07:35 PM
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Re: Twist Rate???

Youre more than welcome.
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  #16  
Old 04-02-2009, 07:38 PM
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Re: Twist Rate???

Yea a 10 twist will stabilize the 240 SMK until around the 1500 yard mark as Bill Baily reported here on LRH.

The higher twist barrels do not affect BC and according to Brain Litz (Berger Ballistician) the only advers affect tp overstabilization is extra spin drift, nothing more. I agree with Brian

Here is Brain's post on Snioers Hide


Bryan Litz
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Registered: 24-08-2008
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Loc: Ohio Ratbert,
Thanks for your comments. Fortunately we're all on the same page at Berger as far as doing what it takes to enhance the shooting experience, regardless of brand.

DB,
Actually the window of proper stability is quite large. On the low end, you have to use at least the minimum recommended twist. On the high end, well, most of the adverse affects cited for over stability are myths. The most adverse effect of very high stability levels is extra spin drift. Other than that, elevation is not affected and BC is not affected.
I think most of the myths about over stability stem from imperfect understanding of the sights including: imperfect zero, click adjustments not being true, canted reticle, etc.
I occasionally see cases of extremely high (and low) BC's being reported by those conducting tests. You'll find that in most of those cases, the non-physical results are caused by oversights in the test.
It's a comfort to know that most of the smoke and mirrors surrounding BC is just that; smoke and mirrors. At the end of the day, actual BC's are very well behaved and constant from all rifles across a large range of stability levels. If two bullets have the same shape and one is heavier, the heavier one has a higher BC. If two bullets are the same weight, and one is more streamlined, the more streamlined one has a higher BC. This is common sense but many 'advertised' numbers contradict these basic trends and it causes unnecessary confusion.

Take care and good shooting,
-Bryan
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Testing for ballistic coefficients - Sniper's Hide Forums
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  #17  
Old 04-02-2009, 07:49 PM
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Re: Twist Rate???

I am not an authority on BC's so I cannot argue with you. However I can point out that Sierra dissagrees with Brian. I have also seen a 9X 308 barrel yeild only a .440 on a 155 AMAX and a 12X barrel yeild Nearly .480 with the same bullets at near the same velocities. Conclusive? Maybe, and maybe not. I wouldnt call it smoke and mirrors though. If youre effceting spin drift, youre also effecting BC since how much spin drift you will get has a direct relation to how much the nose is pointed in the direction of the twist. Increase the angle, increase the drag as well. Am I right or wrong? Not sure. There are many unanswered issues too many shooters have seen.
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Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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  #18  
Old 04-02-2009, 10:27 PM
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Re: Twist Rate???

Hi Guys,

Actually, Sierra agrees completely with Bryan, or vice versa. I shot a series of TOF tests some years ago in both 22 and 30 calibers that took two bullets (.224" dia. 69 and .308" dia. 190 MKs) through a wide range of twists, ranging from excessively fast to completely inadeqate. The 190s were fired in; 1x8, 9, 10, 11, 12 and 14" twists, while the 69s were shot in 1x7, 8, 9, 10 and 12" twists, respectively. The bottom line was, there's very little difference in BC across a wide range of stability factors, until the bullet begins to reach a point of marginal stability. At that point, coning and precession begin to both reduce the measured BCs, and increase the standard deviations and extreme spreads of the sample population. I ran this with Bill McDonald, and he incorporated the results in the Exterior Ballistics chapter that he and Ted Almgren did in the Sierra 4th edition manual. The tests, complete with the scatter plots, velocities and resulting BCs for each twist are reproduced on pages 608-609 of that manual.

As far as negative results from overspinning a bullet, there really aren't any unless you have some jacket concentricity problems, or voids that result in off center CGs. Better to overspin than underspin in virtually all cases.

Kevin Thomas
Berger Bullets
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  #19  
Old 04-02-2009, 11:13 PM
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Re: Twist Rate???

Quote:
Originally Posted by Kevin Thomas View Post
Hi Guys,

I ran this with Bill McDonald, and he incorporated the results in the Exterior Ballistics chapter that he and Ted Almgren did in the Sierra 4th edition manual.


Kevin Thomas
Berger Bullets
Yup. That is where I am getting my info. The difference between 8 and 11 twists was .010. Minimal? Yup. Better? Yup. The manual also states that these tests were done at 150 yards. In my experience an even greater margin can be seen over the long haul.

Thank you Kevin.

I also agree it is better to be over than under but only to a point. Optimum is hard to beat. Of course that is only my opinion.
__________________
Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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  #20  
Old 05-13-2009, 04:05 PM
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Re: Twist Rate???

Quote:
Originally Posted by Michael Eichele View Post
Yup. That is where I am getting my info. The difference between 8 and 11 twists was .010. Minimal? Yup. Better? Yup. The manual also states that these tests were done at 150 yards. In my experience an even greater margin can be seen over the long haul.

Thank you Kevin.

I also agree it is better to be over than under but only to a point. Optimum is hard to beat. Of course that is only my opinion.
Its seems the BC difference is minimal when different twists are fired at the same velocities according to the discussion above. Does anyone have any data indicating the differences in muzzle velocity with various twist rates??

In other words to really compare the performance of two barrels with different twist rates it seems to me external ballistics and internal ballistics should both be considered. Then the slight BC improvement along with a slight velocity improvement may compound to provide a more significant separation in actual performance.

Last edited by 3006savage; 05-13-2009 at 04:06 PM. Reason: added word to sentence
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