Replacement Interchangeable Core Replacement hubs or cores are available in 2 sizes for standard parapets (10″/25cm) or XL parapets (14″/36cm). With 365 Archery's interchangeable center system, it is no longer necessary to replace the entire butt when the targets wear out after a long time of intensive use. Simply remove the old core and slide in a new core for a fraction of the price of a complete new parapet. 365 Archery cores are made from the same proprietary self-healing foam blend that we use in all of our butts and that 365 Archery is known for. It is a material designed to withstand the abuse of broadheads and crossbows.