Charts like those above have some use, but the Berger chart doesnt' mention muzzle velocities or air density. Those numbers can't be optimum for those bullets if fired:
1. In very low temperature air. Faster twists will be requried
2. At very high altitude (mile plus) Less twist is be needed, but a faster twist doesn't hurt much. Excessive spin rate makes bullets stabilze more slowly and have more lateral drift from the Magnus effect, but its much less a problem than understabilzed bullets.
3. at low supersonic velocitites where the bullets are likely to go transonic but not enough muzzle velocity to give them a high spin rate. Worst is around 1200 fps muzzle velocity. Why do it at all is a good question. Maybe for pistol hunting or ???
4. at subsonic velocity (Mach 0.9 or less.) Subsonic drag and supersonic drag are related but not locked together. Few manufatures publish subsonc drag and stability information. Best to just shoot them to find out what works.
The Shilen chart above also doesn't say anytjing about high or low air density. It does say 308 which can be assumed to mean normal 308 loads, but it just gives bullet weight. A 220 grain copper alloy solid VLD certainly needs a faster twist barrel than a 220 grain cast lead round nose The chart says nothing about bullet shape or material.
The charts are useful guidelines, but consider they're based on typical shooters in typical conditons. There are some useful computer programs like McGyro which can assist in determining stability factors of bullets. They're not easy to use as even the internal mass distribution inside a bullet matters. I don't know of any computer programs which can calcuate the stability of bullets at long range as they near transsonic velocities. No manufacture I know of publishes a coefficient for rate of spin loss for their bullets (equivalent to a BC or drag coefficent) but related to the rotational velocity rather than the forwared velocity.
So how do you choose a barrel twist? If you're shooting competition just use what others use as a starting point. For normal range hunting use what others find is succesful in the same area and conditions. If you're pushing extreme ranges where others rarely shoot, the only way I know is to test real bullets in real barrels at the ranges you care about and in the same (or more limiting) atmosphere you care about. Use the charts but temper tham with what you know about the air density. I shoot only at near a mile elevation in Arizona. I never shoot in subzero weather, rarely below freezing. I don't need to worry about how bullets are affected by arctic conditons at sea level. If you shoot in those conditions you're likely to find the charts don't predict a sufficiently fast twist. Likewise if you shoot subsonic or transonic intentionally the charts will be of little use.
