A bullet having a cavity in its forward end that is open at that end and a filling situated within said cavity. Preferably, the filling comprises an elastomer. Preferably, the elastomer filling is vulcanized. More preferably, the vulcanized elastomer filling has a Shore hardness in the range from about 6 to about 90. Alternatively, the filling is a colored rigid polymer filling, if a non-expanding training hollow point bullet is desired. In particular, the invention is a filling and its application to hollow point bullets for the purposes of: improving bullet expansion during the penetration of liquid targets; preventing clogging with debris from intermediate targets; increasing expansion during the perforation of hard materials; preventing the expansion of training bullets; and allowing users to identify different cartridge designs. The invention is also devices and methods for introducing the filling into the cavity.