Marian Brown, 17, was shot dead in disputed circumstances in June 1972.  Her family claim she was killed by shots fired by soldiers.
The witness, know as 'Soldier B', told Belfast Coroner's Court that his Army unit exchanged gunfire with one gunman.
Soldier B said he fired all 20 rounds in his rifle.
A barrister for Marian Brown's family, told the witness: "You emptied your magazine."
She added that his actions were "an exceptional state of affairs".
Ms Brown was pregnant at the time of her death, and during a previous hearing, the inquest was told she had just kissed her teenage boyfriend goodnight before the pair were caught up in gunfire.
On Thursday, the court heard Soldier B was 18 when he first arrived in Northern Ireland, a few months before the shooting.
On the night Ms Brown was killed, he was part of a Royal Anglian Regiment patrol which had set up a vehicle checkpoint in the Roden Street area of Belfast.
The commander of his unit claimed in evidence last week that a number of gunmen fired at the soldiers.
However, Soldier B told the court there was one gunman.
He also told the court that he did not shout a warning.
When asked by a barrister how he felt when he was told that Marion Brown had been killed, he replied: "Not very good."
Soldier B added: "Too many young people were killed out there."
The inquest will reconvene in August.
A soldier fired all his ammunition around the time an innocent teenager was killed in west Belfast in 45 years ago, a coroner's court has heard.