The RCBS Neck Expander Plug is designed for various dimensions including .224 .227 .264 .308 .309 .310 .321 .323 and many others. Regardless of the specific dimension preferred the plug is a replacement part used with the RCBS Neck Expander Die. The primary benefit of using this expander plug is that expansion of the neck and flaring of the case mouth can be completed in one easy step. For reloading cast bullets this is one of the most critical steps so it is vital to choose the best product available which is exactly what RCBS offers.