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For the algebraically inept (me), here is a web-site that will do the math for you.

http://www.shortmags.org/shortmags/ref_data/TwistRateCalc.asp

Also, it is important to note that bullet material may have an influence on the efficacy of Greenhill's numbers. A discussion of this can be found at G.S. Custom Bullet's web site. G.S. Custom

Gerard notes that....

"Gyroscopic stability is what makes a bullet fly in a stable manner. If the variety of conditions governing gyroscopic stability culminate in a gyroscopic stability value of less than one, the bullet is unstable and will fly funny. No amount of lecturing the offending bullet will make it straighten up and fly right. Something has to be changed to bring the gyroscopic stability value to more than one.

Gyroscopic stability is a rather complicated subject and one often hears of the Greenhill formula being used to calculate whether a bullet is stable or not. Using the Greenhill formula is better than nothing, like some makes of chronograph, but for more exact calculations and a true picture, one must turn to the work of one R.L.McCoy. Using the Greenhill formula results in an inconsistent effect on the gyroscopic stability of a bullet if sectional density is changed. The equation does not ask for the right information to accurately take into account varying material densities and forms.

Using McCoy’s method requires inputting the specific gravity of the bullet material as well as a number of form variations that will affect gyroscopic stability. Specific gravity denotes how dense a material is. If specific gravity changes, density changes. This must therefore lead to a change in sectional density! Eureka! A connection! Now we must investigate more thoroughly by making some comparisons to prove the connection.

Still using McCoy’s method, we find that two bullets, identical in form but made from different materials, have different gyroscopic stability values as well as different sectional density values. Changing the sectional density therefore changes the gyroscopic stability. The connection seems to stand. As expected, two identical bullets, made from the same material have identical gyroscopic stability and sectional density values. Not changing the sectional density, leads to no change in the gyroscopic stability and we have two out of two. Now we are cooking! Now we compare two bullets, made from the same material, with identical sectional density values but with different forms, one a semi-wadcutter and the other a spitser boat tail. Alas, they have very different gyroscopic stability values and disprove once again what seemed to be a possible use for sectional density. It is form, rate of twist, diameter and speed that are the big hitters when gyroscopic stability is calculated. As usual, sectional density just tagged along as a coincidental by product of the important stuff."

Best,

John

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