Long Range Hunting Online Magazine


Go Back   Long Range Hunting Online Magazine > Rifles, Reloading, Optics, Equipment > Rifles, Bullets, Barrels & Ballistics

Rifles, Bullets, Barrels & Ballistics Applied Ballistics


Reply

Long bullets require fast twist barrels?

 
LinkBack Thread Tools Display Modes
  #22  
Old 10-15-2013, 09:41 AM
Platinum Member
 
Join Date: Apr 2012
Location: California Central Coast
Posts: 1,389
Re: Long bullets require fast twist barrels?

The short answer is YES.

But

For your older rifles go the other way. Shorter bullets at higher velocity will stabilize.

My 1/12 .223 will only shoot 40grns V-Max and only when pushed as fast as I can.

My German 1960's 270Wby with 1/12 just woke up big time when I switched to 95grns TTSX. They are shorter than the 130grns Accubonds it was designed for.
__________________
Fred Seaman
“Ask, Listen, Learn, Grow”
Reply With Quote
  #23  
Old 10-15-2013, 10:47 AM
Platinum Member
 
Join Date: May 2008
Location: South of Canada and North of Wyoming
Posts: 5,953
Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by Michael Courtney View Post
You would do well to familiarize yourself on some of the stability papers that describe development and testing of the Miller twist rule (for metal bullets) and the Courtney-Miller formula for plastic tipped bullets. See references below. While it is possible to use density and detailed shape information to compute Sg, the Miller twist rule and the Courtney-Miller formula use mass, diameter, and length instead.

Over the years, Don has received various claims that his twist rule has not really worked, but the request for ample follow-up information does not yield sufficient information to determine if the problem lies with an inaccuracy in the twist rule, or a failure in properly determining the inputs or in drawing a conclusion regarding instability from accuracy issues without the bullet actually tumbling.

We do know that in every case where the twist rules have been carefully applied under carefully measured conditions, they have an accuracy of 5% or better in predicting Sg. This is for both plastic tipped bullets, jacketed lead bullets, and solid copper bullets. I've personally been present for testing and analysis of both of non-tipped Barnes bullets as well as TTSX models.

Don was always very interested in following up on purported failures of the twist formula, but very few shooters have been willing to follow up on all the necessary details to properly investigate reported failures. The Litz implementation of the twist rule at the Berger site uses altitude and temperature, whereas Don's original formula uses measured temperature and measured air pressure, but this is at most a minor difference, especially if the altitude is accurately determined. However, the formula does not use density, so any discrepancy cannot arise from the Berger implementation using the density for jacketed bullets.

Courtney, Michael and Miller, Don. A Stability Formula for Plastic-Tipped Bullets: Part 1. Precision Shooting. January 2012a, pp. 47-51.

Courtney, Michael and Miller, Don. A Stability Formula for Plastic-Tipped Bullets: Part 2. Precision Shooting. February 2012b, pp. 79-83.

Litz, Brian. Applied Ballistics for Long Range Shooting. Cedar Springs, MI : Applied Ballistics, LLC, 2009a, 2nd Edition, 2011.

Miller, Don. A New Rule for Estimating Rifling Twist: An Aid to Choosing Bullets and Rifles. Precision Shooting. March 2005, pp. 43-48.

Miller, Don. How Good Are Simple Rules for Estimating Rifle Twist. Precision Shooting. June 2009, pp. 48-52.
Michael, I am not an "expert" on stabilization but I do have a certain amount of basic knowledge and one thing I do know is that the rules are different for copper vs lead core. That's a fact. Monometal bullets of same weight require faster twists than lead core bullets. Even mono's of same length and lighter require faster twists. The twist calc itself bears this out to a degree.

You can also see this in what the bullet makers produce. For instance, Nosler will not make a 308 E-Tip any larger than the 180 gr, reason being, it's a lighter mono and anything heavier and longer than the 180 will not stabilize in an 11" twist. They do make a 200 gr AB which is almost exactly the same shape and will stabilize in the same 11" twist that the 180 E-tips stabilizes in.

In the end, it's not that big of deal. Either your bullet stabilizes or it doesn't. If in doubt, get a faster twist. I have a very good idea of how well certain bullets will stabilize and the Berger calc is a good tool to use. When it comes to mono's, I would subtract bout .2.3 off the calculated Sg. and that will get you close.
__________________
- Mark

You will never know how much it has cost my generation to preserve your freedom. I hope you make good use of it.
~ John Quincy Adams
Reply With Quote
  #24  
Old 10-15-2013, 11:08 AM
Silver Member
 
Join Date: Nov 2010
Location: Southern United States
Posts: 284
Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by MontanaRifleman View Post
In the end, it's not that big of deal. Either your bullet stabilizes or it doesn't. If in doubt, get a faster twist. I have a very good idea of how well certain bullets will stabilize and the Berger calc is a good tool to use. When it comes to mono's, I would subtract bout .2.3 off the calculated Sg. and that will get you close.
The attached graph shows the G1 ballistic coefficient plotted against the gyroscopic stability (Sg) computed with the Miller stability formula for the 53 grain solid copper Barnes TSX in .223 Rem. It is clear that the BC decreases as stability is lowered, but that the bullet is still stable at Sg close to 1.0. Subtracting 0.2 or 0.3 from the stability would imply that the bullet was still stable at Sg = 0.8 or Sg = 0.7 which is clearly nonsense. The bullet would easily tumble and not make it through the far chronograph (or have much more drag) if Sg were really 0.7 or 0.8.

Your suggestion to subtract 0.2 to 0.3 for mono-metal bullets and to go with a faster twist if in doubt may have some basis in your observations. But I expect that the propensity to add more "margin for error" arises from using high volume cartridges that give bullets a big kick in the backside as they leave the barrel, and perhaps a tendency to use bullets with higher dynamic instabilities. We've shot several CED monometal bullets at lower Sg and not seen them tumble.

The standard advice to keep predicted Sg above 1.4 or 1.5 is based on the understanding that most hobbyist shooters are not being careful with the details needed to accurately predict Sg. The margin for error is usually application error, as the formula itself has been shown to be accurate within 5%. The advice to get a faster twist barrel only applies when rebarrelling a rifle and costs about $1000 to apply. Many shooters apply the twist rule to see whether a given bullet is a good choice for an existing barrel and expected environment (shooting match or hunting trip). Careful attention to all the details can yield accurate results with the Miller twist rule, and one can be confident in the outcome with a predicted Sg as low as 1.3.
Attached Thumbnails - Click to View Larger
Long bullets require fast twist barrels?-53-tsx-bc-vs.-sg.png  
Reply With Quote
  #25  
Old 10-15-2013, 11:50 AM
Platinum Member
 
Join Date: May 2008
Location: South of Canada and North of Wyoming
Posts: 5,953
Re: Long bullets require fast twist barrels?

Michael, I read the article you and I believe your daughter published. a very good article and interesting findings.

Back to the mono's, here is a thread in which a member was working with 200 CEB's out of his 300 RUM. They stabilized and shot good groups @ 500 yds and less, and then started to tumble on the way to 600. If you input the data into the Berger calc, you get an Sg of 1.25 Marginal, but still should stabilize with that Sg If you subtract .2 from the calc Sg you get 1.05 which is probably very close to where these bullet were.

200 grain Cutting Edge Bullets
__________________
- Mark

You will never know how much it has cost my generation to preserve your freedom. I hope you make good use of it.
~ John Quincy Adams
Reply With Quote
  #26  
Old 10-15-2013, 11:51 AM
Platinum Member
 
Join Date: Aug 2003
Location: NC, oceanfront
Posts: 3,369
Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by Michael Courtney View Post
It is clear that the BC decreases as stability is lowered
You got it all backwards, it's stability that is decreasing with lower BC -for a given bullet.
Stability is challenged by higher air density, which is analogous to lower effective BC.

You also assign way too much value to the Miller twist rule-of-thumb. It is merely bell curve graphing of typical bullets/twists used today. As you near any edge of this, the results do fall apart, as with any bell curve.
It's a useful tool, but don't confuse it with truths, or 'rules'. It's nothing like that
Reply With Quote
  #27  
Old 10-15-2013, 12:40 PM
Silver Member
 
Join Date: Nov 2010
Location: Southern United States
Posts: 284
Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by MontanaRifleman View Post
Michael, I read the article you and I believe your daughter published. a very good article and interesting findings.

Back to the mono's, here is a thread in which a member was working with 200 CEB's out of his 300 RUM. They stabilized and shot good groups @ 500 yds and less, and then started to tumble on the way to 600. If you input the data into the Berger calc, you get an Sg of 1.25 Marginal, but still should stabilize with that Sg If you subtract .2 from the calc Sg you get 1.05 which is probably very close to where these bullet were.

200 grain Cutting Edge Bullets
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.
Reply With Quote
  #28  
Old 10-15-2013, 12:50 PM
Silver Member
 
Join Date: Nov 2010
Location: Southern United States
Posts: 284
Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by Mikecr View Post
You also assign way too much value to the Miller twist rule-of-thumb. It is merely bell curve graphing of typical bullets/twists used today. As you near any edge of this, the results do fall apart, as with any bell curve.
It's a useful tool, but don't confuse it with truths, or 'rules'. It's nothing like that
The Miller twist rules predict Sg. In 2009, Bryan Litz assigned an expected accuracy of about 10% based on his experience with lots of different bullets. Since then, more careful testing and development of an improved formula to handle the case of plastic tipped bullets shows that the expected accuracy is now as good as 5% for solid metal bullets and plastic tipped bullets.

Larger uncertainties still apply to hollow point and open tipped match bullets due to the empty air space in the front. The Miller twist formula underestimates the stability of these bullets, and if there is a big empty volume in the nose of the bullet, the error can be significant. We are in the process of developing a more accurate formula for open tipped match and hollow point bullets that will use the depth of the open tipped portion to correct the formula for improved accuracy.

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.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes



All times are GMT -5. The time now is 05:48 PM.


Powered by vBulletin ©2000 - 2014, Jelsoft Enterprises Ltd.
Content Management Powered by vBadvanced CMPS
All content ©2010-2014 Long Range Hunting, LLC