A fact that some may know is that the earliest ARs had a 1:14 rifling twist. That was fine at normal temperatures for 55 grain FMJ bullets, but it was later found that it did not stabilize them adequately at very low temperatures. This is because air has greater density at low temperatures, and that will negatively affect bullet stability. The twist was then changed to 1:12, and that proved to be OK at arctic temperatures. However, a 1:12 twist WILL NOT stabilize the later 62 grain bullet (SS109) used in the M855 round, as due to the presence of the steel nose insert in the SS109 bullet, the bullet is more elongated and it will not stabilize. Therefore, the twist was changed to 1:7. Actually a 1:9 twist would be adequate for the M855 SS109 bullet. However the M856 tracer round has an even longer bullet which will not stabilize in a 1:9 twist, therefore the use of a 1:7 twist is required.

The earlier statement is correct, in that for bullet stability, it is the bullet length which is an important consideration. There are some formulas which will estimate the minimum rifling twist to achieve adequate stability for a given bullet length and mass. Surprisingly, the muzzle velocity is not as important to stability as one might think. See: Twist Rate Stability Calculator | Berger Bullets