RCBS pioneered and developed 3-Die Sets for straight-wall rifle and pistol cases. The Carbide Sizer Die sizes the case and de-primes (the carbide sizer ring eliminates the need to lubricate). The Expander Die expands the case to the proper diameter to accept the bullet and it imparts case mouth flare/bell for bullet feeding. The Seater Die seats the bullet and roll crimps.
