A police sergeant is suing the City of London Police for £250,000 after he dodged live ammunition during a firearms training exercise.
Robert Haughey suffered severe post-traumatic stress disorder after shots were fired towards him at Connaught Barracks, in Dover, on 19 March 2015.
A High Court writ says he "feared for his life" on realising that live rounds were being used rather than blanks.
The force said action had been taken "to ensure it could not happen again".
In a statement, the City of London Police said improvements included further training for some of the officers involved.
It confirmed that no-one was struck by the live rounds, and imitation rounds were supposed to have been used.
Mr Haughey, 47, from Strood in Kent, who was not wearing full ballistic protection, realised live ammunition was being fired in his direction when a window was smashed by gunfire behind him.
He "felt panic and terror, feared for his life, and suffered psychiatric injuries as a result".
The High Court writ says he "continues to suffer anxiety and depression, sleep disturbance, flashbacks, intrusive memories and recurrent nightmares", and is "severely depressed" and unable to return to work.
The force admitted liability in October 2017, but no settlement has been agreed.
It said the incident was reviewed by the City of London Police professional standards department in 2015, after which a number of recommendations were made and an action plan implemented.
A subsequent investigation by the Health and Safety Executive highlighted further recommendations, including a full review of the transportation, storage and retention of ammunition and weapons within the force.