The High Court rules today on a bid by a police marksman to challenge a public inquiry finding that he used excessive force when he killed robbery suspect Azelle Rodney.
Lawyers for the officer, known only as E7, say the finding is "irrational and unsustainable".
The marksman is asking two judges for permission to seek judicial review of conclusions reached by the inquiry chairman, Sir Christopher Holland, that Mr Rodney was unlawfully killed - a finding E7's lawyers describe as "tantamount to murder".
E7 shot 24-year-old Mr Rodney in Edgware, north London, in April 2005 when officers stopped the car in which the suspect was travelling with two other men.
The officer opened fire within a second of coming to a halt beside the trio's Golf, hitting Mr Rodney six times, once each in the arm and back, and fatally four times in the head.
Samantha Leek QC, representing E7, told London's High Court at a hearing earlier this month that the Metropolitan Police Service was in possession of reliable intelligence that the men in the car were in possession of machine guns and were on their way to rob Colombian drug dealers. Firearms officers were tasked to intercept them.
E7's justification for firing was that he "honestly believed" Mr Rodney had picked up and was preparing to fire a machine gun capable of firing 1,000 rounds per minute.
Sir Christopher rejected the claim and found in a report last July that E7 used excessive force.
Ms Leek said: "We say it is not a logical conclusion, either on the forensic or eyewitness evidence, that E7 did not honestly believe there was an imminent threat to his colleague's lives."
She said E7's case "has serious implications not only for E7 but also for firearms officers around the country".
She told Sir Brian Leveson, president of the Queen's Bench Division, and Mr Justice Irwin that such cases involved "split-second, life-or-death decisions".
But Ms Leek said: "There are, in the circumstances of this case, conclusions which the chairman reached that will have implications for training and the way in which other police officers approach their task.
"This was a distinguished firearms officer three months from retirement who had received commendations for previous actions in which he had discharged a firearm."
There had been "poor analysis" of the evidence which led to unsustainable conclusions by Sir Christopher, she said.
A single judge, Mr Justice Wyn Williams, has already refused E7 permission for a review.
E7 could face a criminal trial over Mr Rodney's death.
Scotland Yard has backed his application for judicial review which is being opposed by a legal team acting for Sir Christopher.
Following a three-month public inquiry into Mr Rodney's death, Sir Christopher found: "There was no lawful justification for shooting Azelle Rodney so as to kill him. Granted that E7 had an honest belief that Azelle Rodney posed a threat to himself or to other officers, this threat was then not such as to make it reasonably necessary to shoot at him.''
He went on: ''E7's accounts of what he saw are not to be accepted. Prior to firing he did not believe that the man who turned out to be Azelle Rodney had picked up a gun and was about to use it. Further, on the basis of what he was able to see, he could not rationally have believed that.''
Ashley Underwood QC argued on behalf of Sir Christopher that E7's legal challenge was "untenable", and the evidence contradicted "at every stage" his claim that he opened fire because he believed - because of movements he saw Mr Rodney make - that his target had picked up and was preparing to shoot a firearm.