Originally Posted by
TurnipEaterDown
Plain reading, I took this statement to mean that rotational velocity could be computed based on speed of bullet at a point in flight, and that is what I had disagreed with and sought to correct.
What causes a bullet to destabilize?
There are a number of factors.
Forces on the bullet will be non uniform upon Exit from the muzzle shot to shot -- usual clues about this are poor precision capability (aka bad groups).
Uniformity of these forces (or lack of) can be exacerbated by muzzle condition, bullet mass center concentricity w/ geometric center, diameter of bullet base versus muzzle exit ID, etc.
These forces can affect the general stability of the bullet (yaw), and might lead to stability issues, but generally would be more prone to flight path predictability issues unless really bad things were happening. (aka just "bad groups", and not usually a "hey, where did that one go?")
Make these factors really extreme by getting a cast bullet to size a bit off axis, or use ones that have notable voiding or bases not perpendicular to geometric form, and profound lack of stability is nearly assured (the "hey, where did that go", or keyholes in target paper).
With "good" bullets, stability I think often gets challenged as a bullet crosses the sound barrier.
Good LR target bullets (I am thinking about jacketed) seek to have a form that minimizes buffeting effect (uneven force distribution, of a chaotic manner) when a bullet crosses the speed of sound -- dropping below sonic. That, AND / OR the manufacturer Tells You where (given a muzzle velocity) the bullet will no longer stay sonic. (Can be computed with BC & MV.) No accident they tell you this for a factory load, as in sniping. Crossing the sound barrier is rough on flight path control.
I don't know how many cast bullets really take this into effect. I suspect few. I suspect that a notable exception to this is the "good" (i.e. competative) BP Cartridge bullets that are shot at 1000 yds. They start fairly slow. I would believe that they must cross sonic well before the target. Yet they shoot well. The good ones must minimize flight path disruption crossing the sound barrier.
A good indicator to look for on this is: The load shoots well at 50 yd, 100 yd, 150 yd, X yd, with (nearly) linear group size expansion, and then, next yardage mark the group size opens up in relationship to distance. If that is seen, good bet that bullet path has been disrupted, and not just that the shooter had bad habits that show up at long range.
If you ever see this, use a BC calculator for the bullet (some makers might provide), measure MV, and calculate where the bullet goes sub sonic. Likely will be telling.
(BTW: BCs calculated on form are often off. Also, measured may be different from even a good maker published number -- Barnes X bullets used to have optimistic published numbers. Third, if a person calculates the true BC of a projectile shot from a firearm they will likely find it varies shot to shot, and that the same bullet shot from different guns may show a marked difference -- say maybe up to 10%, which is an indication of poor stability (or excess yaw) at the muzzle for ones computing lower than others or very much lower like that 10%.)
Also, just what is stability?
A bullet can precess and still be stable. Precession, as noted, is a yawing of the bullet motion axis along the flight path while in flight.
In fact, to some degree, all bullets precess in flight.
Stability should not be looked at as a bullet purely spinning about its own geometric axis, aligned precisely with the axis of the flight path (technically what many would think of is really the the instantaneous tangent to the flight path). Bullets just don't act like that.
A good quick read on some of this is in the A-Square manual: "Any Shot You Want", pages 117-127, where bullet penetration is discussed.
Bullet rotation keeps the bullet stable while penetrating an object as well.
Distinctly, on page 118 it states: "The bullet retains the vast majority of its rotational velocity throughout its flight."
A final note:
Marginally stabilized bullets (just enough rifling twist) are poor penetrators.
"Over twisted" guns (a significantly higher rate of twist than necessary for flight stability) produce better penetration in game. Why? As the bullet expands its frontal area it requires more spin to remain stable than an undistorted form (Corrected / expanded: initial spin must be high enough that Sufficient spin rate is Maintained to keep the bullet stable during expansion, as conservation of momentum dictates that rotational Velocity will Slow as the bullet expands). It can NOT speed up (edited / expanded, the bullet can not gain rotational momentum), the forces are long gone to do that. It must be spinning fast enough that it Can expand and Still be stable. This would be "over stabilized" for pure flight in air.
A thing I remember being told (older V'Nam era gents w/ knowledge might confirm): Early M16s were provided for combat use w/ twist & bullet weight/form that produced a somewhat "threshold stability" of the bullet in flight, with the Express Intent to have the bullet Destabilize once in the target body, to create a larger wound channel. FMJs don't really expand, a bullet badly yawing or tumbling causes a bigger hole. At least I was told the designers did this.
Must not have worked out too well, I was also told this wasn't kept up too long. Perhaps the twist was increased, pretty sure the bullets were redesigned. Likely both.