It all began with a phone call from an old friend.

Key points: Rodger Muller was setting up a dog food business when he was asked to go undercover for Al Jazeera

Rodger Muller was setting up a dog food business when he was asked to go undercover for Al Jazeera Mr Muller posed as a gun rights advocate to infiltrate the National Rifle Association in the US

Mr Muller posed as a gun rights advocate to infiltrate the National Rifle Association in the US Once the three year investigation was over Mr Muller says he saw a psychologist

Little did Rodger Muller know that answering that call would change his life for three years, leading him to become the head of Gun Rights Australia and cosying up with one of America's most powerful lobby groups, the National Rifle Association.

In his first interview since going undercover for Al Jazeera, Mr Muller has told 7.30 what it was like living a double life.

Why Rodger Muller?

On the phone was friend and journalist Peter Charley, who is the head of Al Jazeera's investigative unit in North America. Mr Muller had not spoken to him in years.

Mr Charley said he was interested in finding out more about the inner workings of the NRA and needed someone to go undercover.

Mr Muller told 7.30 he found Mr Charley's proposition so outlandish he "googled him, because I thought he'd gone mad".

Mr Charley said the two used to live in the same neighbourhood in Sydney and had known each other for over 20 years. They used to drink in the same coffee shop and their kids had gotten to know each other.

Peter Charley says Rodger Muller was the best person he could think of for the investigation. ( ABC News: Jerry Rickard )

"I looked at a number of potential candidates," Mr Charley told 7.30.

"But eventually I thought the best person I can think of is Rodger Muller, because he ticks all the boxes.

"He's got the right temperament and seems to be the sort of guy who could handle a lot of pressure with a good sense of humour."

At the time, Mr Muller was "in the middle of setting up a dog food business", he said.

Becoming a gun advocate

A screenshot from the Gun Rights Australia website created for the Al Jazeera investigation. ( Supplied )

Mr Muller had no undercover or media experience, and Mr Charley had to get him up to speed with handling guns to make him credible as a pro-gun advocate.

"I needed to educate him in that area, so I took him to the US and put him through gun safety courses and gun shooting courses," Mr Charley said.

"I also needed to bring him into the world of hidden cameras."

Mr Charley said he wanted to make sure Mr Muller was properly equipped "because it's a tricky assignment and there's lots of potential pitfalls".

"When Rodger was deployed into the offices of the NRA, he was by himself."

Mr Muller remembers the first time he questioned whether he had made the right decision.

It was his first trip to Washington for a gun safety and concealment course. About a dozen people of "all sorts of walks of life" attended and an ex-policeman ran the course.

"He said, 'bring out your guns', and everybody pulled out a gun from somewhere ... and put them on the boardroom table, and I just basically shrank into my chair with a dozen guns presented in front of me," Mr Muller said.

'There was always a high level of anxiety'

Peter Charley, right, says he felt nervous every time Rodger Muller, left, walked into the NRA building. ( ABC News: Jerry Rickard )

Mr Muller said his first few meetings with the NRA were "quite difficult because of the fear of being caught".

"Every time we had a meeting, we would go there the day before [and] have a look at possible exits," Mr Muller said.

"I was always confident I had someone there from our team, but if we got caught, I was the one getting caught.

"So yeah, there was a lot of anxiety and fear. But as time went on, I got more and more used to the cameras and that people weren't picking them up.

"[But] there was always a heightened level of anxiety and every single meeting was different."

Mr Muller said the fear felt like "you had an anaconda crushing your chest".

Mr Charley said he always felt nervous waiting for Mr Muller to come out of the building.

"When Rodger walked through the door of the NRA and disappeared I would just sit there holding my breath," he said.

Living a double life

Mr Muller said he received "quite a lot of negative feedback" from his community when they believed he was part of Gun Rights Australia.

He said some of his friends thought he was going crazy.

"It was quite difficult because there really wasn't anybody that I could tell," he said.

He also said it was sometimes difficult to remember he was just pretending to be a pro-gun rights advocate, and did form relationships with the people he was meeting in America.

"You start to believe what you're saying, eventually," he said.

"It did become part of my life and quite normal.

"It's hard. And at the end of the day nearly everybody that I met throughout the project were quite nice people, and you do form a bond, because that's part of the job and I think just human nature.

"It was a difficult thing to watch the process."

He said people involved in the NRA are protecting what they "believe is the right laws, and in America being the Second Amendment".

"I was just an average person going in and I've learned a lot through this project," he said.

"The main thing I've learned is how fragile Australian democracy can be and how easily it can be affected."

The fallout

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 41 seconds 41 s Steve Dickson and James Ashby speak to an NRA representative ( Al Jazeera )

The undercover operation dragged One Nation into the headlines after Mr Muller secretly filmed Pauline Hanson's chief of staff, James Ashby, and the party's then Queensland leader, Steve Dickson, meeting with the NRA in Washington.

Mr Dickson, who was also a Senate candidate for One Nation, resigned after secret footage emerged of him making derogatory comments in a US strip club.

New South Wales One Nation MP Mark Latham described the undercover investigation as "media entrapment".

But Mr Charley said he felt comfortable with how Al Jazeera handled the investigation.

"I think a degree of subterfuge is the only way we could have penetrated the NRA," he said.

"The only way to really get deeply into that was to utilise subterfuge, and I'm quite comfortable with that.

"People in America are astonished and intrigued and somewhat gobsmacked by the methods we used."

Back to normal life

Mr Muller has since returned to his dog food business and reconnected with some of the friends he lost at the beginning of his investigation.

But he said he did struggle when it all ended.

He went to see a psychologist "and it really did help to put the job and myself in two totally different buckets".

"It took two or three sessions to convince her that I was telling her the truth. Eventually she believed me," he said.

But he thinks of it as a good story to tell.

"It's something, you know, that not many people get to experience."

Watch Leigh Sales's interview with Rodger Muller and Peter Charley tonight on 7.30.