The former soldier, who cannot be named for legal reasons, shot dead senior Rebels bikie Nick Martin at Perth Motorplex on December 12, 2020 in a sniper-style attack.
The ex-army reservist, 39, told the murder trial of David James Pye, 43, who allegedly paid $100,000 for the hit at the race track, that he stalked Martin before killing him.
"I went to (his) address multiple times and I tailed Nick Martin," he told the WA Supreme Court from the witness box on Thursday.
"I looked for viable points where I'd be able to do maybe a vehicle interdiction and learn some of his routine."
The sniper, who is in prison after admitting his role in Martin's murder, bought a drone from JB Hi-Fi and flew it over the victim's home to inspect its security measures.
He later learned Martin had a race car and "zeroed" in on the motorsport venue, attending a race event where he spotted his target before allegedly reporting back to Pye.
"I told him there are two possible firing locations in order to kill Nick Martin and it can be done," the former artillery gunner said.
"Pye gave me half of the money ... I took the contract that day."
The former part-time soldier, who said he worked for BHP on his return to Australia, used a .308 calibre rifle to shoot Martin while he was sitting in a spectator area at the racetrack.
The bullet travelled through Martin's body and exited from his lower back before striking another man in his arm, seriously injuring him.
"Exactly 365 metres," he said, recalling the long-range shot.
"His entourage of bikies were sitting around him to act as muscle protection."
Pye later messaged the gunman.
"It was the two coffins and a clapping emoji," he said.
"I said two, with a question mark.
"He said one dead, one critical."
The trained sniper first met Pye via Instagram when he was working for a medical charity in Iraq, the court heard.
After he returned to Australia, he allegedly visited Pye at his home in Perth's southern suburbs.
He said he asked Pye to supply him with the illegal drug MDMA, to which the bikie allegedly said he could get a kilogram.
"He was the dodgiest person I knew," he said.
Pye allegedly offered the shooter hundreds of thousands of dollars to murder his ex-girlfriend and dispose of her body on their first meeting.
"He basically just came out and said 'look, the reason I'm under house arrest is because my ex-girlfriend, she's a lying bitch'," the shooter said.
"She's in witness protection. I've found where she is. I'll give you $380,000 to kill her and dispose of her body so it looks like she disappeared."
"I said, I don't kill women. I'm a little bit offended that you would even ask."
He later asked the shooter to kill the Rebels president.
"He said 'I want to kill Nick Martin'," the shooter said.
"He discussed ways that he wanted to kill Nick Martin himself.Â
"Basically just wanted to blow holes in him with a .357 (gun)."
The marksman said the pair were having a dispute over recruitment in the Rebels and their relationship soured when Pye defected to the Comancheros.
Pye has pleaded not guilty to six charges, including murder.
His barrister, David Hallowes SC, earlier told the court the shooter was a liar and the bikie never asked him to kill Martin.
The trial continues.
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