RCBS Pioneered and developed the 3-Die Sets for straight-wall rifle and pistol cases. In the 3-Die Set, sizing and expanding (belling) are done in separate dies. The 3-Die Set has a Sizer Die with Decapping Unit, Expander Die for expanding and belling the case mouth and a Seater Die with Roll or Taper Crimp and Bullet Seater Plug(s) for seating and crimping the bullet.