Cores are most often used to prevent a beaded braid from collapsing. A braid may collapse when the inside of the braid forms too large of a hollow space. A core is used to fill that hollow space and keep the braid round. A 2-drop kongoh beaded braid may develop
2-part epoxy is the only glue that makes a permanent bond and it is the only glue that I recommend for all glue on clasps. Epoxy forms a secure bond between the clasp and the braid even when there is a small amount of empty space in the clasp end.