An assassin who gunned down a bikie boss allegedly at the request of a rival, now on trial for the murder, has recalled the moment he pulled the trigger.

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

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