The latest issue of Precision Shooting (Jan 2012) includes an article that I coauthored with Don Miller where we present an improved stability formula that more accurately predicts stability for plastic tipped bullets. The stability formula that Don published several years ago works well for bullets of near uniform density, but tended to underestimate stability of plastic tipped bullets owing to the much lighter density of the plastic. Consequently, the original twist formula might have caused shooters to lean away from a given plastic tipped bullet design for rifles twist rates and conditions where they are fully stabilized. It was a pleasure working with Don in the development and testing of the new formula, and we expect that the new formula will prove useful.
Improved Stability Formula for Plastic Tipped Bullets
- Thread starter Michael Courtney
- Start date
yeah ok, but what about us blokes that don't get that magazine or don't have the opportunity to read it?
The effect of the plastic tip is usually to change the gyroscopic stability (Sg) by 15-30%. It does not have a big effect on the center of gravity or the moments of inertia, but it does move the center of pressure forward.
The consequence of just using the total length in the original Miller twist rule is that the rule predicts that some plastic-tipped bullets will be unstable when they are, in fact, stable. This will restrict the user to shorter bullets with Sg's much higher than they need to be and leave the shooter with the mistaken impression that some longer, usually higher BC, bullets will not be stable in a particular rifle with given environmental conditions.
The article in PS not only presents the formula, but also discusses a lot of the science behind it. Don Miller's previous articles on stability are also highly recommended background. Here are some links:
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf
The original Miller twist rule has been incorporated into some of the best ballistics calculators and there is also a spreadsheet available. I expect that over time, the improved rule for plastic tipped bullets will be widely available as well.
In my opinion, Precision Shooting is one of the few print shooting magazines worth paying for. The quality of the technical articles tends to be very good, and it's not the same old thing year after year. I learn a lot each month even from the advertisements.
The consequence of just using the total length in the original Miller twist rule is that the rule predicts that some plastic-tipped bullets will be unstable when they are, in fact, stable. This will restrict the user to shorter bullets with Sg's much higher than they need to be and leave the shooter with the mistaken impression that some longer, usually higher BC, bullets will not be stable in a particular rifle with given environmental conditions.
The article in PS not only presents the formula, but also discusses a lot of the science behind it. Don Miller's previous articles on stability are also highly recommended background. Here are some links:
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf
The original Miller twist rule has been incorporated into some of the best ballistics calculators and there is also a spreadsheet available. I expect that over time, the improved rule for plastic tipped bullets will be widely available as well.
In my opinion, Precision Shooting is one of the few print shooting magazines worth paying for. The quality of the technical articles tends to be very good, and it's not the same old thing year after year. I learn a lot each month even from the advertisements.
Just found Mr. Courtney's EXCEL sheet here.... http://forum.accurateshooter.com/index.php?topic=3834705.new#new Hence the 75 grain A-Max stabilizing in many 9tw barrels when the old formula calculated it unstable. Thanks for all your efforts Michael.
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