Bullet Stability Factor

[GrampaMike]

I read where a Stability Factor of 1.5 was best.  Is there a down side if the Stability Factor is higher?   Thanks for any info.

[Questor]

What's stability factor? I usually just go by what the rate of twist  barrel makers decide works best with certain bullet weights for a given cartridge. That's a pretty complex decision for them to make. Most barrel makers, in cases where it might matter, like .223, will specify which twist rate to use depending on your intended bullet weight.

[GrampaMike]

The measure of bullet’s stability in flight is known as the stability factor.  A value of 1.0 describes a bullet that is just barely stable.  If the value drops below 1.0 a bullet is unstable; it yaws wildly.  Accuracy is nonexistent.  It has been determined that a stability factor of 1.3 is the minimum level for reliable accuracy and performance.  Some suggest that the stability factor should be no less that 1.5.  Even bullets of the same weight might have different lengths, so one length may have a better Stability Factor than a bullet of same weight and different length.

Inputs to compute Stability Factor:

Velocity (feet per second).
Length of Bullet (inches).
Weight of Bullet (grains).
Rate of Twist (inches per turn).
Bullet Diameter (inches).

I don't pretend to understand completely, so I was hoping someone here could explain better or even if it matters.

Thanks, Mike

[Questor]

You could try playing with this bullet stability calculator:

http://www.nfa.ca/cfj-archive/ballistics/bullet-stability-calculator-ms-excel-.html

[Davemuzz]

That Excel spreadsheet will never work here!!! You have to shoot your gun in Canada for that to work properly!!!

[GrampaMike]

Thanks Questor, that is almost exactly like something I found. 
But, again nothing says if a SF over 1.5 is Ok or not.
Is it possible to be OVER-Stabilized?
My head hurts.
Mike

[Questor]

This summarizes it nicely, and is consistent with other stuff I've read:
http://en.wikipedia.org/wiki/Twist_rate

[GrampaMike]

Thank You Questor.
Finally something that makes sense and I can understand.
SF is just another factor with Velocity, Length of Bullet, Weight of Bullet, Rate of Twist, Bullet Diameter,
in the search for accuracy.