Denis Reid has given important evidence to the trial identifying Mr Eastman as the man who came into his shop one day and tried to sell him a Ruger 10/22.

Prosecutors allege the former treasury official tried to sell the gun shortly before he used it with a silencer to shoot Canberra police chief Colin Winchester dead. Mr Eastman has pleaded not guilty to murder.

Mr Reid told the court he was "one hundred per cent" sure it was Mr Eastman who had come into his shop.

But the former gun shop owner also admitted he made the "biggest mistake of his life" while later talking to detectives investigating Mr Winchester's murder.

He had told the officers he never had a reason to look at the barrel of the rifle the man had tried to sell him.

But this week he told Mr Eastman's ACT Supreme Court murder trial that when he handled the gun that day he felt the rough thread on the barrel used to fit a silencer.

He said he told the man that he had "rooted the barrel", and that he couldn't buy it because it was illegal and authorities would confiscate his shooting licence.

"It was like a black snake to me," he said.

He said he asked the man to leave his name and number and he would ask his son's friend if he was interested, but the man declined, and instead took Mr Reid's contact details.

Mr Reid and Mr Eastman's defence barrister George Georgiou SC shared several fiery exchanges during a lengthy cross examination on day 16 of the trial, the former at one point saying he felt he was being bullied.

Mr Reid said that if he was put in his "wooden suit" he would still remember the threaded barrel.

"I know you're trying to discredit me but that's okay, that's your job," he told Mr Georgiou.

Questioned by the silk about the statement to police, in which he said he never looked at the barrel to see if it was threaded, Mr Reid said he had made the biggest mistake of his life.

"Maybe it wasn't a mistake?" the silk asked.

"It f---in' was."

Mr Georgiou asked Mr Reid, who was writing notes in the witness stand, if he was writing notes about what was happening to pass onto his son.

"That's bullshit," Mr Reid said.

Mr Reid was interviewed twice by police during the investigation and was asked on both occasions to select the man who had come into the shop from a photo board.

He said he was unable to identify Mr Eastman with one hundred per cent certainty on the first photo board, but on a second photo board months later identified the man.

Later, after police wired him up, Mr Reid went to Civic to see Mr Eastman in person, spoke to him briefly, and then told police he was 100 per cent sure it was the man in the shop.

He said he recognised Mr Eastman's distinct, soft voice.

Under cross-examination, Mr Reid was also grilled on why he did not tell police, or anyone until the coronial inquest in 1990, that the man in the shop had worn a wide-brimmed Akubra hat and glasses.

He agreed that it was possible he had been influenced by images in the media of Mr Eastman in a hat, or the fact that all men in the police photo board he was shown were wearing glasses.

But he denied he was mistaken about Mr Eastman being the man who tried to sell him the Ruger.

He told the court he had tried to avoid media coverage of the investigation, because he didn't want to cloud his memory. He said he did not remember the photo of Mr Eastman published in The Canberra Times.

"I'll walk out of here in a minute," he said, at one moment while being questioned about his memory of the images of Mr Eastman. "Shit."

Returning to a question about whether he'd seen Mr Eastman's image in the paper, Mr Reid said. "I can tell you this and if your ears are painted on I'm not telling you again."

Mr Reid told the court he knew "very well" Mr Winchester's brother Ken Winchester.

He also said he was very close friends with one of the officers who had questioned him with a photo board. He said he would trust the officer with his life.

The trial continues Monday.