David James Pye was convicted of murdering Rebels boss Nick Martin at Perth Motorplex in December 2020.
The 43-year-old is scheduled to be sentenced in the Western Australian Supreme Court on Wednesday.
Pye hired an ex-army reservist Benjamin Luke Johnston to carry out the execution in front of a packed crowd at the popular raceway.
He fired a single shot 365 metres from a .308-calibre rifle, which struck Martin on the chest, causing him to exclaim: "I've been shot".
The bullet passed through the 51-year-old's back and struck his son-in-law, Ricky Chapman, in the left leg before lodging in his arm.
Martin died in hospital but his son-in-law survived.
About 2000 people were at the event and a five-year-old child was seated behind Martin.
Pye later sent the shooter a message consisting of two coffin emojis and a hand clap emoji.
"One dead, one serious," he said.
Pye promised Johnston $150,000 to carry out the hit on his one-time ally.
Their relationship had soured when Pye defected from the Rebels to the Comancheros.
The shooter was given $10,000 to do reconnaissance on Martin who was tailed to his house and had a drone flown over the property.
He scoped out Perth Motorplex on two separate occasions.
Johnston was also convicted of Martin's murder and is serving 18 years after receiving a substantial sentencing discount in exchange for his co-operation.
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