There was a guy years ago who developed and marketed the NECO case neck gage. He died shortly after he brought it to market. I remember reading some of his writings way back then. One of the salient things I remember from his writings is that he purposefully loaded "banana" shaped cases, but oriented them in the same direction in the chamber and obtained tighter groups than those he had neck turned to the same neck dimensions. IIRC, the way he explained it, the non concentric cases being "banana" shaped, that even if the necks were uniform, upon firing and expansion of the cases, they bent the necks and misaligned the bullets towards the thicker sides of the case bodies.
He obtained non uniform groups this way, and uniform groups with the bananas oriented in the same direction.
So, for my own purposes, if I am shooting for groups, I notch the side of my case heads where the case is thinnest, and orient them the same way with the bolt head before I chamber them. It is a whole lot easier than neck turning brass that will bend in a random direction upon firing. Keep the really uniform cases for use in rapid fire 300, and use the bananas for 600.