Improved Stability Formula for Plastic Tipped Bullets

[Michael Courtney]

The latest issue of Precision Shooting (Jan 2012) includes an article that I co-authored 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.

[elkaholic]

Originally Posted by Michael Courtney

The latest issue of Precision Shooting (Jan 2012) includes an article that I co-authored 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.

Interesting! We tend to speak of more twist for heavier bullets which of course is not really true, it is bullet length that is the issue. Along with that is balance of the bullet i.e. weight forward, weight back etc. It would make sense that a lighter tip would have a lesser affect on twist needed to stabilize as opposed to a heavier tip of the same length.....Rich

[Mikecr]

Originally Posted by elkaholic

It would make sense that a lighter tip would have a lesser affect on twist needed to stabilize as opposed to a heavier tip of the same length

How does this make sense?
I know plastic tip bullets are stable beyond predicted, but I don't understand why.

It would seem to me that lighter plastic tip -for a given weight and length of bullet, would move center of gravity backward,, reducing stability.
I might think that the smaller meplat of the tip could move the center of pressure backward, apparently even moreso than center of gravity moved backward. BUT, BC of plastic tipped bullets is no better to worse than typical HP LR bullets. So this Cp notion is not supported with that, because if I form a non-tipped meplat smaller, both BC and stability go up..

What am I missin?

[elkaholic]

Originally Posted by Mikecr

How does this make sense?
I know plastic tip bullets are stable beyond predicted, but I don't understand why.

It would seem to me that lighter plastic tip -for a given weight and length of bullet, would move center of gravity backward,, reducing stability.
I might think that the smaller meplat of the tip could move the center of pressure backward, apparently even moreso than center of gravity moved backward. BUT, BC of plastic tipped bullets is no better to worse than typical HP LR bullets. So this Cp notion is not supported with that, because if I form a non-tipped meplat smaller, both BC and stability go up..

What am I missin?

You may not be missing anything I am not stating scientific fact but merely my own opinion. It is my belief that imperfections are amplified in bullets near BOTH ends. It makes sense to me because the heavier the tip with ANY imperfections, whether ballistic tip or simply the meplat without a tip, the more likely the bullet rotation is likely to throw it off balance. This seemed to happen when I replaced an identically shaped aluminum ballistic tip with a mild steel tip which weighed 15 grs. as opposed to 5. The accuracy went south. I realize this might not happen in every case as there could be different balance issues with different bullets in regard to jacket, core ,etc. The posters theory just seemed to support what I have experienced....Rich

[Michael Eichele]

Deleted

[Michael Courtney]

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/ballist...tability_1.pdf

http://www.jbmballistics.com/ballist...tability_2.pdf

[RBBeers]

JBM Ballistics has graciously added an input for the length of the plastic tip to their stability calculator to reflect the "new and improved" formulas presented by Mr. Miller and Mr. Courtney in their January and February 2012 Precision Shooting articles.

And, although it has not yet been tested and confirmed, for hollow-tip match bullets, inputting the length of the hollow tip (in lieu of the plastic tip) may provide more accurate results than with the original Miller twist rule as well.