Expander Ball
A ball on the sizing die's decapping stem that sets the neck inside diameter as the case is withdrawn, an alternative to a bushing or a separate mandrel.
An expander ball is a hardened ball or button mounted on the spindle of a full-length sizing die, positioned so the case neck is drawn over it as the case is pulled out of the die. The die first squeezes the neck down smaller than needed, then the ball opens it back up to a controlled inside diameter on the way out, which sets the final neck tension. This is the traditional way standard reloading dies size a neck.
The drawback is that pulling the ball through the neck can stretch the case and tug the neck out of alignment, which slightly increases runout. Handloaders seeking more consistency often remove the expander ball and instead control neck diameter with a bushing die or by running an expander mandrel in a separate step. Each approach sizes the neck a little differently, so the method is usually chosen as part of a deliberate reloading process.