If you are encountering "case stretch," it's
unlikely 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.
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** 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 ball.