If you are encountering "case stretch," it's un
likely to be the expander ball** and more
likely the sizing die being screwed down too far for the dimensions of the rifle's actual chamber.
If you are shooting a bolt action, try sizing only enough for the case to barely fit back in the chamber and the bolt close.
** If the expander ball is truly the problem, it will tend to drag the shoulder
forward as opposed to the causing classic case stretch. In either case, get a CARBIDE EXPANDER