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.

Eisner and his Tornante investment group are in exclusive negotiations to pay £5.67m for 100% of the club.
The Pompey Supporters Trust (PST), which controls 48% of the club's current ownership structure, has set out the deal in a 48-page document.
Portsmouth recently secured automatic promotion from League Two.
California-based Eisner, 75, will present his offer in person at a town hall-style meeting on Thursday, and also aims to invest an additional £10m in equity should the deal go through.
The current ownership structure helped bring Portsmouth out of administration in 2013.
Eisner's 70-day exclusivity period runs until 1 June and existing club shareholders will have the final say on accepting any offer.
The PST are set to recommend to its shareholder members an "all cash now" acquisition option, giving them a full return on their original £1,000 investment per share.
Under Eisner's proposals, there would no longer be room for three PST members on the club's nine-strong board.
They would sit instead on a newly-created heritage board to protect matters such as the club's name, colours, badge and moving its home ground more than 15 miles from Portsmouth city centre.
"The PST board believes that the best long-term future for the club is for the PST to retain some ownership," a statement said.
"Tornante have agreed a heritage share can be issued, which will be put in place as a means by which certain identified issues can be protected and vetoes exercised by the PST.
"The decision is for each community shareholder based on their assessment of the offer."
Eisner, who is previously understood to have shown an interest in buying Championship club Reading, is also a former president of Paramount Pictures as well as an early investor in Netflix.
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.
[NEXT_CONCEPT]
Supporters and shareholders of Portsmouth have been presented with the terms of a takeover offer from ex-Walt Disney chief executive Michael Eisner.