Dcsnyder,
Overstabilized just means spinning faster than is optimal. That exaggerates any small imperfections in mass symmetry in the bullet, such as might be caused by slight unevenness in jacket thickness or from a core void. That increases the wobble. It also increases bullet yaw which increases drag a bit.
It is also possible to understabilize a bullet short of going completely unstable. This allows perturbations in the bullet nose that occur at muzzle exit to go underdamped. The bullet path is then actually helical as the nose spirals around the average trajectory position, and may never settle before reaching the target.
There is quite a range of twists over which high quality modern bullets with its nearly perfect mass asymmetry will fly function reasonably well. A gyroscopic stability calculator, like
the one on the JBM ballistics site, will return a number called the gyroscopic stability factor,
s, such that a bullet with a gyroscopic stability factor of 1.0 will be on the edge of instability. Below 1.0 it will be unstable and the nose will describe ever widening circles until the air finally pushes the nose hard enough to cause it to tumble. Above 1.0, the bullet is more and more stable for the reason that the faster you spin a gyroscope, the harder it is to turn it to a different direction.
Sierra suggests
s should must fall between 1 and 3 for rifles, but that is just to function. 1.3 to 2.0 is recommended for best flight by Don Miller and Harold Vaughn feels 1.4 is an ideal compromise between spinning too little and spinning too much, while Miller and a couple of others like 1.5. I've seen a number as high as 1.7 suggested as optimal, which is what the military's preferred 11" twist M14 match barrels get with the old 173 grain match bullet.
Military guns are often set up for more like 2.0 to 3.0 with ball ammo (though they sometimes have had to change when they got it too low, as with the original M16 twist). This is because of the need for light-for-length ammunition, like AP to be stabilized and for ammunition to remain stable in extreme cold anywhere in the world. For example, a Springfield or Garand with 10" twist fires a 152 grain bullet at 2800 fps, giving it an
s=2.9, just a shade under Sierra's recommended upper limit. It still shoots OK.
In the case of your bullets, I found the 120 grain bullets would see about
s=1.9. Not right on the optimal number, but just fine by Miller's 1.3-2.0 critera. A 140 grain CoreLokt seemed to be right at about 1.50 at your likely velocity of 2600 fps or so. Very nice.
Why should that CoreLokt be so ideal when others are saying 140 grain bullets won't stabilize in this barrel? It's because weight and velocity are lesser consideration in stability than length. Length is by far what matters most because length is what provides the lever arm by which air pressure overturns a bullet in flight, making it tumble. Shorter is therefore easier to stabilize and longer is harder.
From the same barrel, a flat base, round nose bullet and a long boattail the same weight, fired at the same velocity, will have dramatically different stability numbers just because of their difference in length. Thus, a .308 150 grain Sierra round nose at 2800 fps only needs an 19 inch twist to fly optimally, while a 150 grain Sierra MatchKing bullet at the same velocity wants a 12" twist for similarly optimal stability.
So, when people say a twist won't stabilize a certain weight, it usually means some bullet or bullets of that weight have not been stabilized by it. When gun writers make the statement, assuming they are cognizant of the fundamentals, then it is a generalization that bullets of this weight typically have lengths great enough that this twist won't stabilize them. Not that there are necessarily no bullets of that weight that the gun can stabilize.
In case of your 140 grain CoreLokt, it is simply short enough that it does well with the twist you have. It's about 0.1 inch shorter than a Sierra MatchKing of the same weight. Where your 10" twist is about perfect for the CoreLokt, the MatchKing wants a 9" twist to get the same
s value. With the 10" twist I get
s=1.2, so it should not be unstable, but won't be inside Don Millers optimal range. A Berger 140 grain VLD will be 0.2" longer and will be unstable in the 10" twist barrel (s=0.97) and would require an 8" twist to be optimally stable.
Some will suggest just increasing velocity to increase stability, but the effect is much less than length or twist change produces. This is because even though the greater velocity makes the bullet spin faster, it also pushes the bullet nose into the air faster, so that it needs that extra spin to be stable against the additional air resistance. The latter doesn't fully cancel out the former, but the net stability increase produced is modest. For the Berger bullet to achieve s=1.50 in a 10" barrel like the CoreLokt has at 2600 fps, the calculator says it would have to be driven at close to 10,000 fps; not something it would survive even if it could be done.