Jimro, your comment:
If there chamber is perfect, and the brass perfect, and no extractor pressure, then gravity would be pulling the top and compressing the bottom, so we would expect more expansion on the top.
... won't happen. The front of a rimless bottleneck case has exactly the form and shape as the front of the chamber. Doesn't the front end of a headspace NO GO gauges coned shoulder center perfectly in the chamber's coned shoulder when the bolt tries to close on it?
The case shoulder will perfectly center in the chamber shoulder when the firing pin makes the round fire; 98% of the case Body doesn't touch the chamber wall at all. At the back end, it could be anywhere, but often is pressed against the chamber wall by the extractor if it is putting pressure against the case at its extractor groove. If there's no force applied at the extractor the back end of the case could center most any place, especially if it's twisted by contact with the bolt face when the bolt closes or gets moved off center by the firing pin striking the primmer off center on the primer's anvil driving it sideways a bit before firing.