Home Forums Hunting & Shooting Ask The Gunwriters Miller Stability Value

This is an interesting subject that takes a lot of digging and thinking to comprehend. I researched it myself to find the answers, and the best search term to use is "tractability." Tractability is the term used to describe the bullet's ability to keep its rotational axis tangent to its trajectory.

It may be true that overspinning a bullet can cause issues when a bullet is not perfectly dimensioned and constructed. The bullet might disintegrate, or simply exhibit degraded accuracy, but this is not the same thing as overstabilization.

A bullet that is tractable in flight keeps its rotational axis tangent to the trajectory and impacts the target [almost] perfectly nose first. An overstabilized bullet is not tractable.



Tractability must be maintained throughout the bullet's flight. If it isn't, the bullet does not fly [almost] perfectly as it begins to descend on its trajectory. If tractability isn't maintained, then it begins to exhibit a slightly nose-up attitude and that would negatively impact its BC at that time and the trajectory becomes no longer predictable. Your dope is no longer valid at that point.

Tractability is a product of the interaction between gyroscopic force and the overturning moment. The overturning moment is produced by the center of pressure caused by aerodynamic resistance, and is constantly trying to disturb the relationship between the path of trajectory and the rotational axis of the bullet in flight, but the gyroscopic force resists the overturning moment. The reason that it works is that the gyroscopic force reacts to the overturning moment by countering it with a 90-degree offset response. During the instant that the overturning moment wants to push the bullet's nose up, the gyroscopic force pushes the nose either to the left or the right, depending on the direction of spin (CW or CCW). It's a constant battle between the two forces. In simplest terms, the overturning moment is always "a day late and a dollar short" but its presence allows the bullet to fly perfectly [almost] nose-first.

I do not know how far you'd have to shoot to see the effects of overstabilization, but I'm pretty sure that it would have to be a lot further than most of us shoot. I am not sure just how fast a rate-of-twist would have to be to induce overstabilization, either. It just doesn't seem to happen to the extent anyone would notice it but it's certainly possible. Somewhere I read about someone experimenting with twist rates as quick as 1:4. Don't ask me where I saw that because I sure as hell couldn't recall.

The reason that too fast a rotation can theoretically exist is that forward velocity degrades much, much faster than rotational velocity. At some point, this would cause the overturning moment to become so weak that it cannot affect the gyroscopic tendency of the bullet to maintain its current attitude. This would mean that the overturning moment would no longer have the ability to force the bullet to correct any discrepancy between the bullet's rotational axis and its trajectory.

Here's an interesting video about the .408 Chey-Tac. I've watched it a few times, and what I get out of it is that having the correct balance between gyroscopic force and overturning moment at the time the bullet encounters the trans-sonic zone is critical in long range shooting. This is not stuff I employ, or ever plan to make use of, but always wanted to understand. I just like to know (when and if I can).