Slightly expands the case neck so that the bullet can be seated easily.Expanding (or Belling) Die
Plastic storage case
Straight wall cases require an extra die to expand the case mouth. The case mouth needs to be slightly flared to allow seating of the bullet.
This die is usually founf as the second die in a set for straight walled cases.
Bottleneck cases get "expanded" as the case is drawn over the expander ball while being removed from the resizing die.