Why does bullet stability and shootablity change so much with a change in bullet material .
Because copper weighs less than lead and therefore a 150 grain all copper bullet is the same length (aproximately) as many 165 or 180 grain lead core bullets.
As I write I see that maybe it has more to do with the relationship of twist rates to bullet density than having to do with shape .
You need to know the length of the bullet and the velocity at which it will be fired, and nothing else, to calculate what twist rate you need for a given bullet.