Larry if you would take a shot at poking holes in the following I would appreciate the feedback.
I think I understand twist rate a little better. At least on the heavy/length end of the spectrum. The feedback I got from barrel and bullet makers was focused on the twist rate with heavy bullets and there is little said about light weight bullets and their optimum twist rate. Shilen said that a one in nine twist is marginally acceptable with a 70 grain bullet. That means that a 72 grain bullet being marginally unstable with a one in 9 twist. NOE confirmed this. They said that a 70 grain should stabilize but that you might have to play with the velocity to get it stabilized.
Berger has a calculator on their web site for their jacketed bullets that lets you to play with twist, weight and velocity and it spits out a “you’re ok”, or “you’re not ok” for stability. It’s called the Stability Factor and carries the acronym SG. Greater than a 1.5 is considered stabilized with the maximum SG achievable being 2.49. Variables such as bullet weight, twist rate and velocity can be played with to see how the stability optimization is effected. It can be found here:
https://bergerbullets.com/twist-rate-calculator/
Example: With a one in 9 twist the 73 grain Berger BT @ 3,000fps has a SG of 1.47 and it is considered marginally unacceptable. The same twist, bullet at 3,200 fps has a SG of 1.51 and just barely acceptable for stability. But at 1,700fps with the same bullet and twist the SG tumbles to 1.22 and is unstable in the one in 9 twist (remember this is programmed for jacketed, not cast).
Example: With a one in 7 twist the 73 grain jacketed @ 3200fps has a SG of 2.49 and at 1,700 fps the SG is 2.02 with the faster velocity excellent and the slower velocity pretty good stability.
So stability is tied to both weight/length and velocity. The slower the twist the faster the bullet has to be pushed to stabilize. Within reason (above 1,400fps) the faster the twist the slower the bullet (ie cast bullet) can be shot and still reach stability.
This is somewhat supported by the CBA Benchrest results sheets showing that the very few folks using .223’s were shooting 8 twist and bullet weights of 78 to 84 grain @ 1.600 to 1,900 fps. So I'm comfortable in having the .222 barrel coming in with an 8 twist for 70 grain cast bullets. As a dedicated heavy cast bullet shooter I would go for one in 7. Since I'll be splitting time with 52 grain match jacketed bullets I'm hedging my bet and hoping that the eight twist will work for me.