Component of the cylindrical core that assembles on the inner surface of the capsid during capsid formation and plays a role in viral DNA ejection into the host cell. The inner core is composed of stacked rings of gp14, gp15 and gp16 proteins. Following binding to the host cell surface, the internal core is disassembled and gp15 is ejected along with gp14 and gp16 into the infected cell. Gp15 probably remains associated with gp16. The gp15-gp16 complex binds to both the viral DNA and the host inner membrane, probably escorting the leading end of the genome through the periplasm and controlling the extend of DNA translocated into the host cell. Homooctamer (PubMed:12614603). Interacts with gp16; after ejection the gp15-gp16 complex composed of a gp15 octamer and a gp16 tetramer probably binds both the viral DNA and the host inner membrane (PubMed:26476287). Interacts with gp14. The gp15-gp16 complex spans the periplasm and the cytoplasmic membrane. Belongs to the T7virus internal virion protein gp15 family.