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Long bullets require fast twist barrels?

 
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  #8  
Old 10-06-2013, 10:59 AM
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Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by trazman View Post
I know that. How do you calculate the twist for monometal bullets?
You might try this one. You need to enter the specific gravity for mono's. Good luck

http://www.border-barrels.com/barrel_twist.htm
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  #9  
Old 10-06-2013, 01:39 PM
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Re: Long bullets require fast twist barrels?

Actually, the Miller rule of thumb will work with solids, as uniform specific gravity is not needed given weight of a bullet.
Border's calculator assumes weight of the bullet is not known, as it's a bullet design tool.
Notice no bullet weight input field.

Both will pass and fail tests. I believe Borders comes from a bullet design tool written by Bill Davis, using much of Robert McCoy's math.
McCoy's approach represents more truth than rules of thumb can, but is difficult in use as many actual/difficult measurements must be taken for it.
The miller rule of thumb is useful for typical bullets in use today, as it was with Greenhill a long time ago. And it fails tests with bullets on the edge of todays 'typical' scale. But it's way better than Greenhill.
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Old 10-06-2013, 08:49 PM
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Re: Long bullets require fast twist barrels?

Thanks to all of you who commented on my original post. There were several good points made. For one, it seems that a bit more twist than you need is better than a little less twist than you need. However, Iím still looking for an answer to my main question. I should have been more specific. So let me be and maybe that will help get to the crux of the situation.

The bullets Iím having trouble with are the heaviest/longest Barnes LRXs available in several calibers, namely the 7mm, the 30 cal and the 338. Has anyone shot repeated 0.5 MOA groups with any of those LRX bullets and if so what bullet and what twist were you using? And if you donít mine what was your chambering, load, jump, barrel length and velocity.
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  #11  
Old 10-06-2013, 09:34 PM
RTK RTK is offline
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Re: Long bullets require fast twist barrels?

I shoot the barnes 200lrx out of a 300wm, 26" rock creek 1:10 twist running a little over 2900fps.. Before the last 500+ rounds down the tube I was .040 off the lands, now I am about .070 off with no real difference in group size but a little slower confirmed by drops today.
When I do my part I can keep them under .5moa at 100yrds.
The last time out I was running a tad over .5 vertical at 750yrds shooting bi-pod prone with my fist and styrobead bag at the rear.
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Old 10-06-2013, 09:36 PM
Edd Edd is offline
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Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by Mikecr View Post
But if you shoot short flat base bullets with a twist producing Sg of ~1.5, or even ~1.8, they will still shoot fine,
What if the Sg is 2.8?
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  #13  
Old 10-06-2013, 10:25 PM
RTK RTK is offline
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Re: Long bullets require fast twist barrels?

Here's a pretty good explanation of rpm and over stabilization.

Long Range 101 Part 69 - Bullet RPM & Overstabilization - YouTube
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  #14  
Old 10-07-2013, 09:02 AM
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Re: Long bullets require fast twist barrels?

Engineer- it is more a question of barrel quality. if you go to riflemansjournal, German shoots several different twist 30-06's. 10, 11, 12. generally the long bullets are a little more finicky, but my 7remmag shoots 168 bergers much better than the 140's even though it is shorter. also if you go to 6mmbr.com; the 6mmbr with an 8 twist ( fast ) shoots the light bullets great also; as well as the heavier ones.
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