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 Taper Crimp and Bullet Seater Plug(s) for seating and crimping the bullet.