The RCBS 15503 is a two-die set, specifically designed for bottleneck type cases. These cases are particularly used in tight-chambering semi-automatic, as well as pump or lever-action rifles. The set includes a Small Base Sizer incorporated with an Expander-Decapping Unit, along with a Regular Seater Die equipped with a Seater Plug.The Small Base Sizer functions to resize cases below Saami minimums, resulting in a reduced shoulder and body by a few thousandths. It also possesses the ability to de-prime the case. Correspondingly, the accompanying Seater Die contains an in-built roll crimper for bullet securing simultaneously as it\'s seated. For those desiring to seat bullets without crimping, this can be simply achieved by unscrewing the die until the crimp ring no longer comes into contact with the case mouth.