The woman constable was cleaning her service rifle when she accidentally shot herself in her right thigh, SP Vikas Varman said.
The incident took place hours before a training camp at a firing range near police lines, he said.
The injured, a native of neighbouring Nalanda district, was rushed to Patna Medical College and Hospital (PMCH) for treatment, Varman said.
A probe has been ordered into the incident, the SP added.