RCBS pioneered and developed 3-Die Sets for straight-wall rifles and pistol cases. These sets for straight wall pistol cases build off of that legacy. Each set features a steel Sizing die. The Expander die expands the case to the proper diameter to accept the bullet and also imparts case mouth flare/bell for easy bullet feeding. The bullet seating die then seats the bullet and puts on the appropriate crimp for the round being loaded.