Yes, if you are resizing the case, you will need the Decapper in the die
as the Decappers play a part in the sizing process. If you are resizing
with the Collet Neck Sizing Die, the sizing Collet needs to squeeze
against the Mandrel. If you are using the Full Length Sizing Die, the
Die Body sizes the case down too small, while the expander ball on the
Decapping Rod opens the case neck back up.