Lt Cdr Ian Molyneux was killed trying to tackle Ryan Donovan on HMS Astute in Southampton

Southampton Coroner's Court heard that Lieutenant Commander Ian Molyneux would have fallen unconscious immediately after being gunned down by able seaman Ryan Donovan on HMS Astute.

Donovan, 23, was jailed for life to serve a minimum of 25 years after pleading guilty at Winchester Crown Court to the murder of Lt Cdr Molyneux on April 8, 2011.

The navigator yeoman also pleaded guilty to attempting to murder Lieutenant Commander Christopher Hodge, 45, who he shot in the stomach.

The crown court heard that his real targets, who he also admitted to attempting to murder, were Petty Officer Christopher Brown, 36, and Chief Petty Officer David McCoy, 37.

Donovan's attack was only stopped when the then leader of Southampton City Council, Royston Smith, and its chief executive, Alistair Neill, wrestled the weapon from him.

The inquest which resumed at Southampton today heard that Lt Cdr Molyneux, 36, suffered a single gunshot wound to the top of his head, six inches above his right earhole.

Home Office pathologist Dr Basil Purdue said: ''It would have caused instantaneous unconsciousness followed very, very shortly by death.

Royston Smith alongside HMS Astute Royston Smith alongside HMS Astute

''The lieutenant commander would have known nothing about it.''

The inquest heard that Lt Cdr Molyneux was heard to be breathing, making a ''snoring'' sound, as he lay on the floor after being shot.

Dr Purdue said: ''It's upsetting and emotionally distressing that someone horribly injured is still breathing but they are beyond help at that stage.

''As soon as he received that wound, he would have been out of it completely.''

Dr Purdue said that the position in which Lt Cdr Molyneux was found lying face down on the floor was consistent with him rushing forward to tackle the gunman.

He added that, at 6ft 2in, Lt Cdr Molyneux was a tall man and would have had to lower his head forward in order to suffer the injury to the top of his head.

He said: ''It's not an unreasonable presumption that he's throwing himself, rushing forwards, moving towards him with his head down.''

Ian Molyneux and his wife

Dr Purdue added that because of gunpowder residue found on the injury, it would have been suffered at very close range.

Mark Mastaglio, a forensic firearms expert, said that from tests to replicate the residue, the shot would have been fired 5cm from the wound and the rifle would have been fired from waist level.

He said that the weapon used was a SA80 military high velocity rifle which fires rounds at 940 metres per second and when in automatic mode can fire 800 rounds a minute.

A total of seven shots were fired during the incident, the inquest heard.

The crown court sentencing hearing was told that Lt Cdr Molyneux, a father-of-four, known as Molly, had bravely tried to tackle Donovan after hearing previous shots from the SA80.

His widow Gillian, who attended today's hearing, has said previously that nothing could ever replace her soulmate and father of Jamie, Arron, Bethany and Charlie and ''the heartbreaking sadness for the loss of Ian''.

Donovan had been drinking while ashore and had volunteered for guard duty when onboard, because he admitted he was intent on killing the two petty officers who had reported him for disobeying a direct order to clean a part of the sub.

Able Seaman Ryan Donovan Able Seaman Ryan Donovan

Donovan had been told he would not be leaving the sub for an attachment on another vessel because of his behaviour, and he had anger towards them, the crown court heard.

The amateur rapper, who called himself Reggie Moondog, was given the weapon and 30 rounds by PO Brown.

He turned the gun on him and CPO McCoy but the men dived for cover or fled.

Then weapons officer Lt Cdr Molyneaux, whom Donovan had no grudge against, turned up and a witness heard him say ''What have you done?'' seconds before he was shot.

The crown court heard that Donovan then moved into HMS Astute's control room which was full of local dignitaries visiting the vessel on a five-day goodwill visit to Southampton's Eastern Docks.

Horrified onlookers saw Donovan come with the gun at waist height and shoot Lt Cdr Hodge with a ''wild'' expression on his face.

Donovan was then wrestled to the ground by Mr Smith and Mr Neill and the gun went off harmlessly for the seventh time during the struggle.

When questioned by detectives, Donovan said: ''I just felt that everyone was out to get me.''

In mitigation, Christopher Parker QC said that Donovan, from Dartford, Kent, was immature and not mentally ill but he could not handle stress.

Sentencing, Mr Justice Field called the shootings a ''murderous onslaught''.

The inquest, which is being heard by a jury, was adjourned until tomorrow.