If you are encountering "case stretch," it's un
likely 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.
** 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