Jeff Siewert

There's a normal, what's called a normal force center of pressure location.

And that's forward, it better be forward of the bullet center of gravity, okay?

So that separation between the CG and the center of pressure location in calibers multiplied by another arrow coefficient called the normal force coefficient gives us this CM alpha, okay?

And it's...

You can think of it as stability, okay?

The bigger you make the CM alpha, the smaller you make the gyro stability, okay?

And we'll talk about the effects of dispersion on another podcast break, okay?

So, but looking at the spin squared to velocity squared for a given rifle or given handgun, given weapon,

That ratio of spin to velocity is fixed because the rifling.

The only way you can really change the gyroscopic stability is by changing the velocity and how that affects the pitching moment coefficient derivative.

That's a function of the Mach number.

Air density is in there as well.

The higher the air density, the lower the stability.

From a design standpoint,

That's a gyro stability of 1.4 to 3.5 in most atmospheric conditions.

It's typical with an air density of 1.2 and the units are kilograms per cubic meter.

When a bullet enters high density material like water or tissue, that material density is a thousand times larger than

than the atmosphere.

So your gyroscopic stability is much, much less than one.

And that's what causes the bullet to tumble when it enters high density material.