Re: bullet twist rates over velocity?
If there is an imperfection in the bullet, then there can be two types of imbalance, static, and dynamic.
Imagine a perfect bullet, then imagine adding a small lump of mass near the front, on the left side. Add another at the back, also on the left side. This is statically imbalanced, you can detect it while the bullet is motionless. If you rotate the bullet on its axis, it will be heavier on the side where the extra masses are. The axis that the bore forces the bullet to rotate around does not pass through the center of mass. Once free of the bore the bullet flies off it's path slightly.
Dynamic imbalance occurs if the mass at the back were on the right side, then it would counter the mass at the front during static measurements. Once the bullet is spinning fast, the mass at the front would pull opposite to that at the back. This twists the bullet from flying straight along its trajectory.
Both effects are bad for accuracy. The different densities of copper and lead, and any deformation of the core and jacket cause these imbalances.