It would be difficult for a gun linked to the deaths of campers Russell Hill and Carol Clay to go off accidentally, a police forensics officer has said.
Former airline captain Greg Lynn, 57, has pleaded not guilty to two counts of murder over the deaths of Mr Hill, 74, and Ms Clay, 73, in Victoria's alpine region and is on trial in the Supreme Court in Melbourne.
It's alleged Mr Hill and Mrs Clay died on March 20 after an argument between Lynn and Mr Hill at Bucks Camp in Wonnangatta Valley.
Earlier in the trial, Mr Lynn's barrister Dermot Dann said his client had admitted his involvement in the pair's accidental deaths.
He said a scuffle broke out after Mr Hill took a gun from Lynn's vehicle and Mrs Clay was accidentally shot by Mr Hill.
The barrister said Mr Hill then came at Lynn with a knife and was accidentally stabbed in the chest as Lynn defended himself.
However, prosecutors allege Lynn intentionally killed Mr Hill and Mrs Clay.
Victoria Police officer and ballistics expert Leading Senior Constable Paul Griffiths told the jury about his investigations into projectiles found at the campsite and a gun seized from Mr Lynn's home.
Sen Const Griffiths undertook testing on the 12-gauge Barathrum Arms shotgun, taken from Lynn's home, including a safety function test and trigger pull test.
A trigger pull test measures the amount of force that has to be applied to the trigger to fire a shot.
Lynn's trigger pull was 3.947 kilograms, slightly above the industry standard of between 1.8kg and 3.6kg.
"Being higher, means have to pull trigger harder to discharge the gun," Sen Const Griffiths told the court on Tuesday.
He also tested whether the gun would fire if dropped or if it was overloaded.
"It failed to discharge ... which tells me the firearm is in good working order," he said.
During further questioning by defence barrister Mr Dann, Sen Const Griffiths said he undertook trajectory testing, using string testing from the car, through a side mirror to where Ms Clay was shot.
He said those line tests measured the lowest and highest points the bullet could have tracked through the hole - this was to test if the bullet would have hit Ms Clay if she was crouching or standing.
Another test of the middle ground was done, but Sen Const Griffiths did not take photos or notes of that test.
He conceded that the middle measurement meant Lynn's allegation that Ms Clay was next to Mr Hill's 4WD when a shot passed through the side passenger mirror and struck her in the head could be "plausible".
In a fiery exchange, Mr Dann suggested that Sen Const Griffiths had not done enough to ensure he had all the information he needed to undertake fair trajectory testing.
"I only had limited information, that's all I had," he said.
Earlier, Sen Const Griffiths admitted he had made an error in a report previously provided to the court.
That report said he had used Lynn's firearm for trajectory testing when he had instead used a 12-gauge shotgun from the Forensic Services Centre library.
"I put it in my statement erroneously," he said.
"It was an oversight ... it had no impact on the result of the exercise."
The trial continues.
