Long bullets require fast twist barrels?

[stomp442]

Keep in mind also that the stabilization factor increases with range as the velocity of the bullet decreases faster than the rate of spin. So if you have marginal twist and (SG) at the muzzle you will actually have better (SG) at 400 yards. This is why its always important to test loads further down range. A load that produces a 1" group at 100 yards is very capable of producing that same 1" group at 300 yards.

 

[Mikecr]

So we just go lower in twist and our bullets will self-correct back towards targets?
Awesome!
Wait a minute.....

Ok,, you almost had me there stomp..

 

[Michael Courtney]

That twist calc is a good calc for lead core VLD bullets but not for monometal because of the difference in material density.

False. The Miller twist rule works well for monometal bullets. It works better for bullets with constant material density than jacketed lead bullets where the jacket is slightly less dense than the core.

 

[MontanaRifleman]

False. The Miller twist rule works well for monometal bullets. It works better for bullets with constant material density than jacketed lead bullets where the jacket is slightly less dense than the core.

Sorry Michael, The calc that is on Bergers site is for the specific gravity of lead core bullets. That's why it's there. It makes no sense for them to provide a calc that is based on the specific gravity of copper monos. I've used that calc for the 6.5 140 CEB's. Another member, Outlaw 6.0, also use the calc and figured he could get the 140's to stabilize inhis 8.4 twist. He could not. If memory serves me right (and maybe he will chime in) the inputs were about a 3000' elevation, 40* and 3200 fps or more. The Sg the calc gives is 1.31 which although marginal, is enough to stabilize a bullet.

 

[Michael Courtney]

Sorry Michael, The calc that is on Bergers site is for the specific gravity of lead core bullets. That's why it's there. It makes no sense for them to provide a calc that is based on the specific gravity of copper monos. I've used that calc for the 6.5 140 CEB's. Another member, Outlaw 6.0, also use the calc and figured he could get the 140's to stabilize inhis 8.4 twist. He could not. If memory serves me right (and maybe he will chime in) the inputs were about a 3000' elevation, 40* and 3200 fps or more. The Sg the calc gives is 1.31 which although marginal, is enough to stabilize a bullet.

The density is NOT a required input for the Miller twist rule. The calc on Berger's site does not depend on the bullet density. It depends on the length, weight, and caliber to be accurately entered, as well as the twist rate and atmospheric conditions. We've used the twist rule to accurately predict stability of several Barnes X bullets.

One always needs to ask what the evidence was that the bullet was not stable. Less than desired accuracy is NOT evidence of instability. High speed video of tumbling bullets or keyholes in paper targets is convincing evidence of instability. Inaccuracy can have many other causal factors. Careful determinations of BC vs. muzzle velocity can indicate marginal stability.

The numbers you cite above would require the bullet to be no longer than 1.5". What's the actual bullet length for the CEB 140 grain 6.5? Also, be mindful that a cartridge that shoots a 6.5 140 grain bullet at 3200 + fps is going to have an abnormally high pressure when the bullet leaves the barrel. Such a large pressure on the base as the bullet leaves the barrel is going to be more likely to induce tumbling than normal cartridges. The twist rule predicts stability for a bullet in free flight, with no additional forces than air drag. The twist rule does not predict stability when subjected to significant additional forces.

Finally, be mindful that the predicted stability depends strongly on an accurate twist rate. If the actual twist rate were 1 in 8.6" rather than 1 in 8.4", the Sg would be reduced from the predicted 1.31 to 1.25. A 1 in 9.0" twist rate would produce an Sg of 1.14. I would recommend that twist rates actually be measured. Trusting gunsmiths or manufacturers can lead to inaccurate results.

 

[MontanaRifleman]

The density is NOT a required input for the Miller twist rule. The calc on Berger's site does not depend on the bullet density. It depends on the length, weight, and caliber to be accurately entered, as well as the twist rate and atmospheric conditions. We've used the twist rule to accurately predict stability of several Barnes X bullets.

One always needs to ask what the evidence was that the bullet was not stable. Less than desired accuracy is NOT evidence of instability. High speed video of tumbling bullets or keyholes in paper targets is convincing evidence of instability. Inaccuracy can have many other causal factors. Careful determinations of BC vs. muzzle velocity can indicate marginal stability.

The numbers you cite above would require the bullet to be no longer than 1.5". What's the actual bullet length for the CEB 140 grain 6.5? Also, be mindful that a cartridge that shoots a 6.5 140 grain bullet at 3200 + fps is going to have an abnormally high pressure when the bullet leaves the barrel. Such a large pressure on the base as the bullet leaves the barrel is going to be more likely to induce tumbling than normal cartridges. The twist rule predicts stability for a bullet in free flight, with no additional forces than air drag. The twist rule does not predict stability when subjected to significant additional forces.

Finally, be mindful that the predicted stability depends strongly on an accurate twist rate. If the actual twist rate were 1 in 8.6" rather than 1 in 8.4", the Sg would be reduced from the predicted 1.31 to 1.25. A 1 in 9.0" twist rate would produce an Sg of 1.14. I would recommend that twist rates actually be measured. Trusting gunsmiths or manufacturers can lead to inaccurate results.

Michael, with all due respect, specific gravity of the bullet most certainly plays a role in determine stability. a bullet of a particular shape and size with more mass will stabilize with a slower twist rate bore than one with less mass in the same shape and size. That is a natural and physical fact. I'm sure you know that. Other factors include, center of gravity and canter of pressure. Things like plastic tips vs non plastic tips will also affect stability. With the Berger bullets, Their form and densities are close enough to give a good "ball park" stability factor by just inputting the length, cal, weight and enviro factors.

The 6.5 140 gr CEB is 1.513" in length.

I can't comment on your use of the calc for the Barnes bullets, but maybe you can provide the inputs? If they were tipped, their pressure point and center of gravity would be more favorable to a slower twist for their length.

 

[Michael Courtney]

Michael, with all due respect, specific gravity of the bullet most certainly plays a role in determine stability. a bullet of a particular shape and size with more mass will stabilize with a slower twist rate bore than one with less mass in the same shape and size. That is a natural and physical fact. I'm sure you know that. Other factors include, center of gravity and canter of pressure. Things like plastic tips vs non plastic tips will also affect stability. With the Berger bullets, Their form and densities are close enough to give a good "ball park" stability factor by just inputting the length, cal, weight and enviro factors.

The 6.5 140 gr CEB is 1.513" in length.

I can't comment on your use of the calc for the Barnes bullets, but maybe you can provide the inputs? If they were tipped, their pressure point and center of gravity would be more favorable to a slower twist for their length.

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.

 

[Deleted member 46119]

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.

 

[MontanaRifleman]

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.

 

[Michael Courtney]

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.

 

[MontanaRifleman]

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.

http://www.longrangehunting.com/forums/f19/200-grain-cutting-edge-bullets-110316/index4.html

 

[Mikecr]

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

 

[Michael Courtney]

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.

http://www.longrangehunting.com/forums/f19/200-grain-cutting-edge-bullets-110316/index4.html

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.

 

[Michael Courtney]

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.