The mysterious assassination of Rebels boss Nick Martin in front of thousands of people at a family event sent shockwaves through Western Australia and sparked fears of a bikie war. 

A single shot was fired from a distance of 335m, piercing Martin's chest before lodging into former Bandidos bikie Ricky Chapman at Perth Motorplex on December 12 last year, prompting a theory that a professional killer was behind the attack. 

As the months went on and no arrests were made, Perth remained on edge. 

But what people did not know was that police had a very strong lead that they were chasing. 

A crucial mistake by the sniper meant police were able to gather evidence and build a case against him. 

The killer, who cannot be identified, had taken his rifle to a licensed armourer in January and asked for the barrel to be replaced. 

He then collected the rifle less than one week later, the West Australian Supreme Court was told this month. 

The following month, police attended the armourer's premises and seized the rifle barrel. 

Forensic testing confirmed projectiles fired from it were consistent with the bullet removed from Chapman. 

Investigators further established that the man was at the motorplex at the time of the shooting and specifically at the location from where the shot was fired. 

He was identified as a suspect and placed under surveillance; the court was told. 

In March, the 35-year-old was arrested and charged with murdering Martin. 

During a sentencing hearing last week, Justice Stephen Hall described Martin's murder as a "contract killing". 

"Offences of this nature are not only serious in themselves, but they strike at the heart of our system of law and order," he said. 

"The safety of all members of the community depends on lawful authority being respected. A contract killing is, in effect, an unlawful public execution. 

"The importance of public denunciation and general deterrence is particularly important for an offence of this nature." 

The man became a suspect after he took his rifle to an armourer. Photo / Facebook 

Justice Hall also noted Martin's friends and family – including his wife, stepdaughter and a child – witnessed his murder. 

"You knew when you fired that his wife was seated next to him. The trauma that they experienced was entirely predictable," he said. 

Justice Hall further determined that the most important mitigating factor in the case was that the man was willing to testify against Rebel turned Comanchero bikie David James Pye, who allegedly orchestrated the killing. 

Pye is yet to plead to multiple charges against him, including the murder of Martin. 

He allegedly paid the sniper to thoroughly research and watch Martin in preparation for the murder and also allegedly offered him another $600,000 to kill someone else although that never eventuated. 

Justice Hall noted the sniper had provided a detailed witness statement and signed an undertaking to give evidence at any trial of Pye. 

"Your future promised co-operation is of great importance in ensuring that others who may bear some responsibility for what occurred are brought to justice," the judge said. 

"This assistance places you at risk from those who would seek to prevent you from giving evidence or who seek retribution for you having done so. 

"This will mean increased hardship because you will be kept in a highly restrictive environment. 

"There can be no doubt that your co-operation falls into the exceptional category. 

Nick Martin, pictured here with his wife Amanda, was killed in a public shooting. Photo / Supplied 

"A substantial discount is due not only to provide an incentive to you to comply with your promise but also to make it known to others that co-operation will be rewarded where it serves the greater public interest." 

The man avoided a life sentence and was instead jailed for 20 years. He will be eligible for parole after serving 18 years behind bars. 

If he fails to co-operate with authorities as promised, he will be resentenced to life in prison with a minimum of 30 years. 

Defence lawyer David Manera indicated to reporters outside court that his client might consider an appeal. 

Meanwhile, the heartache endured by Martin's family was also evident in court during the sentencing hearing, as one person walked out in tears and his father angrily held up a fist at the sniper. 

Justice Hall said the family members spoke "powerfully of the shock and grief" they each felt at Martin's death in their respective victim impact statements. 

Martin's daughter Tia described him as her best friend. 

She had recently given birth to a baby named Niki, in honour of her father, and he had been spending a lot of time with them. 

She said she continued to feel numb and thought about her father every day. 

Martin's mother Rhoda Grimes said her whole world had collapsed and the way he died would haunt her for the rest of her life. 

She described Martin as a trusted and reliable rock in the life of her and the family. 

Martin's father Sidney said there were no words to describe his pain, adding his family had been destroyed. 

He also said he was experiencing a living nightmare and had lost the will to do things. 

Martin's sister Renee said she felt flat, rarely saw friends and mostly stayed at home. 

She said she thought her brother was invincible and she did not believe he would die when she was told he had been shot. 

Martin's wife Amanda described him as her whole world, saying he was a kind, generous and funny man. 

They were together for 11 years and had plans to retire and enjoy peace and quiet together, she said. 

Without Martin she said she felt vulnerable and feared for her safety. 

Martin's stepdaughter Stacey Schoppe said the things she witnessed on the night of the shooting remained with her. 

"Whatever opinion others may have had of Mr Martin, it is clear that he was greatly loved by his family and they have been devastated by his murder," Justice Hall said. 

- by Angie Raphael, news.com.au