Entry wounds made by small-calibre ammunition generally ranged from 8mm to 11mm, Alexander Dempster said.
But the wound to the top of 18-year-old Laniet's head had a 15mm entry point, with the injury to the underlying skull extending to 25mm because of two large bone fragments bevelling inwards.
The damage was possibly enhanced by being "a tight contact wound", with the rim around the silencer pressing hard against the scalp.
That created an increased amount of pressure, Dr Dempster said.
And, with silenced weapons, the velocity of the bullet decreased but the energy transfer tended to be greater.
This caused a massive pressure effect within the skull and that would be transmitted to the brain "with relatively quick fatal effects to the brain stem".
Dr Dempster's clinical and at times harrowing evidence came at the start of the fifth week of 37-year-old David Cullen Bain's second trial for the 1994 murders of his parents Robin (58) and Margaret (50), sisters Laniet and Arawa (19) and younger brother Stephen (14).
All five were shot to death in the family's Every St home in Dunedin on June 20, that year and the then 22-year-old student and sole survivor, David Bain, was charged with the murders and found guilty at his trial a year later.
Two years ago, the Privy Council directed the matter should be heard again.
Bain denies responsibility for the killings.
He says he returned home from his early morning paper round and found his family all dead.
His defence is that his father shot the four other members of the family then committed suicide because he was about to be exposed by Laniet for sexual offending against her.
The Crown says Robin Bain could not have shot himself and a brief of evidence read to the court yesterday related to the question of the length of Mr Bain's arms.
Bespoke tailor Ivan Coward used Robin Bain's dinner jacket and a photograph of him wearing the jacket to make the estimate.
He put the full span of Robin Bain's arms at between 185 and 190cm, the inside length of his arm at 45cm in a hanging position, with an outstretched length of 51cm to 52cm.
During his graphic evidence about the way the five Bain family members died, Dr Dempster said he had not been able to do core body tests to try to establish the time of death because he was not brought into the house until about six hours after the shootings.
But he said tests to determine the time of death were not necessarily accurate.
The bodies were all cold to the touch although Margaret Bain's was warmer because she was lying on a water bed and Robin Bain's was also not so cold because he was wearing warm clothing.
Robin Bain was on the floor of the front room with a rifle nearby and an ammunition magazine lying on its back edge.
There were several blood splatters or spots on his clothing and blood from a gunshot wound to an area near his left temple had run down on to several items of the clothes he was wearing.
The silencer of the rifle was extensively smeared with blood and there was blood on the rifle barrel.
Dr Dempster also described several small blood spots on Robin Bain's fingers.
And he told the court of hearing a sound from a curtained-off alcove while he was checking the body.
He asked police officers what it was and, because he was nearest to the curtains, carefully parted them and looked in the alcove where he saw a computer running and a line of typing on the screen.
That was the message "You are the only one who deserved to stay".
His approach at scenes was to "keep my hands to myself and not do anything to disturb the scene", Dr Dempster told Crown counsel Robin Bates.
"But in this case, it was so extraordinary, and I was the closest to the curtains . . . ," he said.
He told the jury about seeing the body of Margaret Bain lying in her bed in a fairly natural-looking sleeping position.
She had been shot through the left eye socket.
During the postmortem examination of her body, he found she had survived long enough to have inhaled some blood, as both lungs were severely congested.
An indentation on the duvet on the bed and the upward angle of the bullet entry suggested the killer may have put a knee on the bed before firing the gun.
The body of Stephen Bain was lying on the floor of his bedroom.
He had been strangled with his own T-shirt to the point where he was unable to fight, then fatally shot through the top of the head at the end of a violent struggle.
A wound to the left hand suggested a bullet had passed through the hand then creased along the top of the scalp before lodging in a pillow.
That laceration wound to the head would have "bled vigorously", Dr Dempster said.
It would have accounted for much of the blood in the room "but it wouldn't have disabled him".
Arawa's body was lying on the floor of her bedroom, her legs bent beneath her.
She had died from a single gunshot wound to the head, but when he carried out the autopsy on her body, he found her lungs were slightly congested, suggesting she may have survived for a brief time.
Laniet had three bullet wounds, one to her left cheek which would have incapacitated her, one in the area above her left ear and one to the top of her head, either of the latter two being potentially fatal.
Dr Dempster said he believed the wound to the cheekbone occurred first because Laniet had blood on her hands.
She had obviously been able to move her hands to try to discover what had happened.
She may have recovered consciousness or might not have lost consciousness.
He did not think she would have been in a position to do that after either of the other two wounds: "She would have been immediately incapacitated".
The amount of fluid in her lungs suggested Laniet survived for some time after the first wound, which was not a contact wound, unlike the "unusual" wound to the top of her head.
