Police forces and legal chiefs are meeting in London to discuss how they deal with violent confrontations which could lead to lethal force being used.
Armed police in London are more common in London since 7 July
It comes after criticism of London's Metropolitan Police by the family of a Brazilian man shot dead when he was mistaken for a suicide bomber.
Pc Mark Williams, of the Met's SO19 armed unit, said criticism after the shooting had "taken its toll".
The event was organised by officers' group, the Met Police Federation.
Pc Williams, who is the federation delegate for CO19, said the last thing an officer would want to do was shoot a person.
Mr Menezes was shot a day after the failed London bombings
He told BBC reporter Zubeida Malik: "It is a split-second decision these officers are making that lives with them for the rest of their lives. Some of them never recover."
He said it was "not acceptable" that investigations into police shootings sometimes take up to six years to complete, adding that officers felt "left out to dry".
But he said support from senior officers for the 497 officers in CO19 had improved in recent years.
American law enforcement expert Bill Lewinski, who will address the conference, said "not a lot" goes through an officer's mind when they make the decision to shoot.
He said: "It would be the equivalent of you driving down the road and suddenly having a car pull out in front of you - what would go through your mind?"
Officers from around the UK, delegates from the Independent Police Complaints Commission, barristers and other legal professionals have been invited to the conference.
Jean Charles de Menezes, a 27-year-old electrician, was shot dead the day after the failed 21 July bombings in London.
Met Police Commissioner Sir Ian Blair has received sustained criticism from the Menezes family for allowing flawed information to be released to the media in the shooting's aftermath.