Met Commissioner Sir Bernard Hogan-Howe told an audience at the Royal United Services Institute (RUSI) that firearm officers needed greater public support.
He also revealed that despite a push for more armed officers, there were not enough people willing to do the role.
Sir Bernard is retiring after more than five and a half years in the post.
During the speech, the Met's commissioner said public pressure on firearm officers had meant "we simply don't have enough people now wanting to do these jobs".
"This a dangerous place to be... we can't afford to have officers think twice because they fear the consequences of shooting someone".
"That's how they get shot or the public gets hurt or a criminal gets away with a gun," he said.
Sir Bernard also stated that armed officers did not fire a shot at a suspect in more than 3,300 incidents involving firearms in 2016.
"It's an urban myth that officers are trigger-happy," he said.
The Met has said it will increase firearm officer numbers by 600, with more than 400 extra staff trained or selected to date.
According to a staff survey, nearly half of Met Police officers believe more officers should carry guns.

The 22-year-old played for Yeovil in the 2012-13 season and briefly last term, but has been studying at Rider University for the past four years.
"I'm incredibly excited to be back playing for my hometown club who I have supported my whole life," she said.
"To have the opportunity to play for them in the top division is amazing."
Howard graduated two weeks ago with a major in history and a minor in French.

It found almost half a million people have had to leave work for medical reasons within five years before they were due to retire.
In March the government announced an independent review into the state pension age.
Research by the TUC research points to a significant north/south divide.
In the south west of England, just one in 13 people blamed sickness or disability for leaving work. However, that figure rises to one in seven in the north of England, Scotland and Wales and one in four in Northern Ireland.
Those in the lowest-paid jobs or in manual work are also twice as likely to stop working for health reasons than managers or professionals.
TUC general secretary Frances O'Grady said: "These figures show that we must hold off on any further rises in the pension age until we have worked out how to support the one in eight workers who are too ill to work before they even get to state pension age.
"People should be able to retire in dignity with a decent pension when the time is right. Older workers have a crucial role to play in the labour market but we can't expect the sick to wait longer to get a pension when they may need financial support more than ever."
A government review of the state pension age is being led by ex-CBI chief John Cridland. Its findings are due to be published in May.
The state pension age is set to rise to 65 for both men and women by November 2018 and 67 by 2028.
Tom McPhail, head of retirement policy at financial services firm Hargreaves Lansdown, said earlier this year: "Those joining the workforce today are likely to find themselves waiting until their mid-seventies to get a pay-out from the state system."
Britain's most senior police chief has called for "less suspicion and more trust" in officers who carry guns, in his final speech before retirement.
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Women's Super League One side Yeovil Town Ladies have re-signed goalkeeper Beth Howard after she graduated from university in the United States.
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Ill-health or disability is forcing one in eight people to stop working before they reach the state pension age, the TUC says.