Armed siege barrister Mark Saunders put his shotgun into a non-firing position as he was killed by a volley of police bullets, an inquest has heard.
Firearms expert Robert Griffiths examined his 12-bore Beretta shotgun and footage of the 32-year-old's death at the hands of Metropolitan Police firearms officers.
He concluded the gunman switched a lever to open the breech of the weapon after he was hit by the first of five bullets fired by police surrounding his Chelsea home.
Westminster Coroner's Court heard it is impossible to say whether the action was intentional as the family law specialist collapsed with catastrophic injuries.
The inquest heard the shotgun did not contain any ammunition when it was recovered but any cartridges may have fallen out as the weapon opened. Mr Griffiths, of the Forensic Science Service, said used and unused cartridges were found strewn across the floor of the Markham Square property.
Under cross-examination by Patrick Gibbs QC, who represents Mr Saunders' widow Elizabeth, the expert said the gunman's hand could be seen close to a lever that opened the weapon.
Mr Gibbs said: "It looks as though he was either opening or just at the moment of opening the gun when he was shot." Mr Griffiths replied: "That is correct, but I cannot say what his intentions were or were not at that time."
Mr Gibbs added: "Of course, we are all geniuses after the event, but in the open position the gun could not fire."
Forensic experts blasted a slab of pork belly with a shotgun from 23 metres to find out if Mr Saunders could have killed any of the officers near his kitchen window. The inquest heard pellets penetrated through the meat, chosen because of its similarity to human tissue, suggesting any injury could have been fatal.
Alcoholic Mr Saunders was killed after sparking a five-hour stand-off with police when he fired his gun through a window during a solitary drinking binge on May 6, 2008.