The L.E. Wilson Expanding Mandrel Sizing Die is designed to be used on NEW brass cases to prepare the case for the seating operation. Over the past few years we have seen a greater need to run new brass cases over and expanding mandrel to properly prepare them for their first firing. New brass cases are often damaged when shipped in bulk, this die will give you a consistent neck diameter on your cases. With correct neck tension you can expect a more consistent seating depth and shot.Note: Includes one mandrel. Additional mandrels and die sold separately.