Originally Posted by MontanaRifleman
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.