I don't know what "case setback" is, but from the discussion I gather it is a tendency of a shouldered cartridge to fill the shoulder in the chamber upon firing and push the base back against the recoil shield? Please educate me.
If so, I wonder if this is why the old Colt SAA cylinders were notorious for having the shoulders cut so far forward that case life was short due to the subsequent amount of necessary case resizing. Perhaps having space ahead of the case shoulder prevents "case setback"?