The model he used in the tests was about the same height as Robin Bain but his arms were slightly shorter, UK ballistics export Philip Boyce said.
If the person had shot himself in a standing position while leaning forward, he would have expected the person to fall backwards, Mr Boyce said.
Contrary to what was shown in Hollywood films, a person would not be thrown across the room by the force of the shot but would crumple and fall backwards, he told the court.
Other demonstrations using the model in several positions showed he was easily able to reach the trigger with his thumb or with a finger.
Evidence from a ballistics expert called by the Crown, using the murder weapon with a model wearing the helmet showed the person had great difficulty reaching the trigger.
The Crown says it was David Bain who shot his parents and three siblings and tried to frame his father, Robin, but the defence says Robin killed his wife and three of his children before turning the rifle in himself.
Mr Boyce said Laniet Bain's body had been moved.
The shot to the top of her head could not have happened in the position in which she was found, so he believed she would have been upright, and that would have been the first shot.
He had the notes, and worked with a blood splatter analyst who will be called as a witness by the defence, to decide which was the first shot.
He was aware of the presence of white fibres on the bullet fragments found in Laniet's skull.
Mr Boyce said the bullet hit something else before it hit the top of her head.
He had no idea what it hit, but the fibres were white cotton.
Mr Boyce's evidence is continuing.
Earlier Sharleen Stirling, a friend of Arawa Bain, said the Bain family were very welcoming, a lovely family.
She said Margaret Bain and Arawa had a normal mother-daughter relationship, except when they had an argument. Then Mrs Bain wouldn't talk to Arawa, until she apologised to her.
Mr and Mrs Bain were tense sometimes, when she was there, and they didn't talk to each other.
She said knowledge was wealth in their home, and Arawa didn't like talking to her father.
Ms Stirling said Mrs Bain stayed in bed a lot, and Arawa found that hurtful especially when she did it on Christmas Day.
Darren Palmer, a meter reader in the area of Taieri Beach School, told the court that in early June 1994, he went to the school, and while on his way to put the key for the schoolhouse back in the office at the school he met Robin Bain.
He said a man (Robin Bain) stopped him. He had his hands in his pockets and his head down. He did not look at Mr Palmer while he was talking.
He asked him to do a final reading for the schoolhouse, because the name on the account was under Bain, and he wanted to put the power back under the board of trustees' name.
Mr Palmer said the man told him he was going back to town. He spoke very quietly and came across as quite depressed, not someone involved with kids. He had expected someone more out-going.
