Marginal Stability
A stability factor only slightly above the threshold, which can shave BC and open groups, with the effect strongest in cold, dense air.
Marginal stability describes a load whose stability factor sits only a little above the value needed to keep the bullet flying nose-first, leaving almost no safety margin. A bullet in this band may print acceptable groups in warm weather yet trim its effective ballistic coefficient and string vertically as conditions change. Because spin decays slower than velocity, a marginally stable bullet can fly straight up close and still degrade at distance.
The effect grows in cold, dense air, since lower density altitude raises the air’s resistance and pushes a borderline bullet toward instability or even keyholing. The fix is a faster twist rate or a shorter bullet that lifts the stability factor into a comfortable buffer. Most experienced shooters target a healthy margin rather than the bare minimum, which keeps performance steady across changing conditions.