If you are encountering "case stretch," it's
unlikely to be the expander ball** and
more likely the sizing die being screwed down to far for the dimensions of the rifle's actual chambere dimensions.
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 expanded ball is truly the problem, it will drag the
shoulder forward as opposed to the causing classic case stretch. In either case, get a .
http://www.midwayusa.com/product/547731/hornady-carbide-expander-spindle-assembly-for-rcbs-dies-219-zipper-222-223-22-250-remington-220-swift