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Long bullets require fast twist barrels?

 
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  #29  
Old 10-15-2013, 03:23 PM
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Re: Long bullets require fast twist barrels?

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Originally Posted by Michael Courtney View Post
You are confusing Sg (gyroscopic stability) with dynamic stability. If a bullet has gyroscopic instability, it will tumble within the first 50 to 100 yards. A bullet which flies well until 500 yards, but tumbles before 600 yards is demonstrating dynamic instability. See Chapter 10 of Applied Ballistics for Long Range Shooting.

Increasing the twist rate may or may not improve dynamic stability issues. It may be that (unlike most bullets) this specific bullet is losing angular velocity faster than it is losing forward (translational) velocity. Something else may be going on.

If these bullets really had a Sg between 0.95 and 1.05, I expect the increased drag would be creating problems long before the bullet reached 500 yards. I expect the Sg is close to 1.239 (you should use the true diameter of .3075") and that dynamic instability is the source of the problem.
In reading the 10th chapter as you suggested, Litz described dynamic stability as a type of stability that gets a bullet through the transonic zone. This isn't the case with the above example. Not sure what's going on there as I realize in theory that a bullet should become more stable the farther down range it goes. The guys at Cutting Edge did tell him to possibly expect that.

I'll be running them in my 300 RUM at higher elevations and velocity. It will interesting to see how they do.
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  #30  
Old 10-15-2013, 04:26 PM
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Re: Long bullets require fast twist barrels?

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Originally Posted by MontanaRifleman View Post
In reading the 10th chapter as you suggested, Litz described dynamic stability as a type of stability that gets a bullet through the transonic zone. This isn't the case with the above example. Not sure what's going on there as I realize in theory that a bullet should become more stable the farther down range it goes. The guys at Cutting Edge did tell him to possibly expect that.

I'll be running them in my 300 RUM at higher elevations and velocity. It will interesting to see how they do.
From Litz 2009 p. 160 (Emphasis in the original):

It's common knowledge that bullets fired with adequate gyroscopic stability can become unstable at long range, tumble, and keyhole in the target. Most shooters assume that the problem is a lack of gyroscopic stability, and sometimes try to remedy it by using a faster twist barrel. A faster twist barrel certainly does increase the gyroscopic stability of the bullet both at the muzzle and downrange. However, when a bullet tumbles at long range, it's not because of a lack of gyroscopic stability, but through a lack of dynamic stability.

At Litz goes on to explain, this effect most commonly is seen somewhere around the sonic transition, but there are cases where lack of dynamic stability causes tumbling at long range even though the bullet is nowhere near the sonic transition. The dynamic stability condition is complex (see: http://www.nennstiel-ruprecht.de/bullfly/dynacond.htm ). Even though it may not immediately be clear in this case why the bullet becomes dynamically unstable, it is clear that the dynamic stability rather than the gyroscopic stability is the problem. A faster twist barrel may not fix the problem. All of the terms in the dynamic stability equation are characteristics of the bullet itself. Increasing the gyroscopic stability (with faster twist) is hopeless if the dynamic stability becomes equal to one and the dynamic stability condition becomes impossible to fulfill.

If the bullet has dynamic stability problems, faster twist rates, higher elevations, and higher muzzle velocity can (at best) push the onset of dynamic instability out to longer ranges. But if the problem is already appearing at 600 yards, I would be surprised if the bullet can ever be made to shoot reliably at 1500 yards.
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  #31  
Old 10-16-2013, 10:13 AM
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Re: Long bullets require fast twist barrels?

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Originally Posted by Michael Courtney View Post
Once formulas are proven to make accurate predictions within 5-10%, I think they need not be referred to as "rule of thumb" and can be applied with confidence within their expected uncertainties.
Truths pass all tests.
Your rule of thumb will fail tests as long as bullet makers/experimenters are adjusting center of gravity -vs- center of pressure.
For instance, the current Miller rule of thumb results would not predict well with VLDs fired base first.
McCoy's math certainly would, provided you're inclined to understand and use it.
Still, McCoy's math holds flaws, and assumptions. So we currently hold no truths beyond carefully field validated.

This is why I see current efforts to bell curve typical bullets as useful,, in a Greenhill sense.
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  #32  
Old 10-16-2013, 11:07 AM
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Re: Long bullets require fast twist barrels?

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Originally Posted by Mikecr View Post
Truths pass all tests.
Your rule of thumb will fail tests as long as bullet makers/experimenters are adjusting center of gravity -vs- center of pressure.
For instance, the current Miller rule of thumb results would not predict well with VLDs fired base first.
McCoy's math certainly would, provided you're inclined to understand and use it.
Still, McCoy's math holds flaws, and assumptions. So we currently hold no truths beyond carefully field validated.

This is why I see current efforts to bell curve typical bullets as useful,, in a Greenhill sense.
Of course, the claim of the Miller and Courtney-Miller twist formulas is an expected uncertainty of 5%. This means the standard deviation of the bell curve is 5% of the mean. In contrast, the standard deviation of the bell curve for the Greenhill formula is more like 50%. The claim is for usefulness, not for absolute truth. The measure of usefulness for empirical formulas is their uncertainty, not their absolute truth in the sense of exactness with no errors.

To date, the published Miller and Courtney-Miller twist formulas have proved to be accurate within 5% for all applicable cases for which experimental data is available. Don's original papers (2005 and 2009) include data validating the accuracy of his twist formula in 14 different bullets. Since 2011, we've collected data carefully validating the accuracy of the twist formulas in 11 different bullets.

In Don's 2009 paper, he pointed out the inapplicability of the original twist formula for plastic tipped bullets. We fixed that problem with an improved formula published in 2012. We are currently tweaking the formula to reduce the uncertainty in cases where it applied to open tipped bullets with a significant air space in the front.

The point about firing bullets base first is somewhat academic with almost no practical application. However, we have actually fired a number of bullets base first and can report in all cases we've tried, bullets fired base first are actually more stable in flight than those fired nose first. There are some practical challenges in loading and shooting bullets base first, but if one overcomes these difficulties, we expect that a successfully launched bullet will remain in stable flight (base first) if the formulas predict stable flight nose first.
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