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Long bullets require fast twist barrels?

 
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  #15  
Old 10-07-2013, 10:27 AM
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Re: Long bullets require fast twist barrels?

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.
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  #16  
Old 10-07-2013, 06:45 PM
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Re: Long bullets require fast twist barrels?

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..
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  #17  
Old 10-14-2013, 10:35 PM
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Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by MontanaRifleman View Post
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.
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  #18  
Old 10-14-2013, 11:09 PM
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Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by Michael Courtney View Post
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.
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  #19  
Old 10-14-2013, 11:52 PM
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Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by MontanaRifleman View Post
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.
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  #20  
Old 10-15-2013, 12:21 AM
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Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by Michael Courtney View Post
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.
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  #21  
Old 10-15-2013, 09:24 AM
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Re: Long bullets require fast twist barrels?

Quote:
Originally Posted by MontanaRifleman View Post
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.
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