The crime occurred in a deserted field at Bidupur in Vaishali, about 40 km from the state capital.
"Two girls were gang-raped at gunpoint. The girls were returning to their village after collecting wood for fuel when the assault occurred," a district police official said.
Police have registered a case on the basis of the statement of the victims. A medical test has confirmed the sexual assault.
"The girls told us that the accused threatened to kill them if they informed the police," the official said.
All five accused have been absconding since the incident. Police have still to make any arrest.