Originally Posted by
DreadPirateMoyer
Correct. You are either gyroscopically stable or you're not. There's no in between (excluding external factors like wind, bullet construction [for example, a flaw in the copper jacket can be exacerbated as the bullet spins faster], and other things). I believe MistWolf's comment was more in regards to the bullet barely being above that stability threshold, meaning as it slows down, it will hit the non-stable point faster than it would in a faster twist barrel.
As for the bullet being more stable as it slows down, I don't see how that's true. Maybe less affected by external factors, but as far as gyroscopic stability, regardless of if the bullet loses forward velocity faster than rotational velocity, it's still losing rotational velocity, and that's the only variable in the equation with a bullet of static mass/size/etc. A loss in rotational velocity will eventually destabilize the bullet as it crosses between the threshold of gyroscopically stable and not. The forward velocity doesn't affect that.