devildoc, I think thats as good of a way to see it as any out there.
And if you think about it, it's right there in front of us..
An 8:1 twist barrel is causing the bullet to displace 8" per turn, as released from the muzzle.
8" of dense air that a completely unbalanced bullet has to traverse, without tipping. If it were balanced better, like a yard dart, it could get by without spinning at all.
Down range the bullet has slowed, but RPMs have not really, so the bullet displaces less air per turn. It's effective twist rate increasing all the way, and so is it's gyroscopic stability factor. Except, drag is changing downrange as well. With most supersonic bullets drag goes up as the velocity slows(peaking at mach1). So even with less and less displacement per turn downrange, that air seems denser to the bullet. This counters increasing stability and with a -very good- bullet, Sg downrange climbs at a reasonable rate instead of going through the roof(described as 'over-stabilization'). There is dynamic stability that comes into play here, and some of our bullets are not -very good- in this regard(like a 155smk).
But I know of no accurate predictions for dynamic stability. It's usually found through testing(with radar).
There is additional wiggles to all this and I sure as hell don't have a firm grasp on them. But stability directly comes down to displacement(not time or speed), for that turn to overcome.
How does that single turn overcome it?
I don't remember gyroscopic theory or even application anymore. I just look at it now like that turn in 8" is enough to keep the bullet from knowing which way to tip, given it's desire to do so. In 12" it may figure it out, no matter what!