A Supreme Court jury took less than three hours to find Mark Raymond Hinchliffe not guilty of the attempted murder of his former colleague Scott Williams, 44, at his North Yunderup property on May 7, 2011.
The jury of five women and seven men found Hinchliffe guilty of the alternate charge of grievous bodily harm with intent.
Two female relatives of Hinchliffe's cried after the jury delivered its verdict.
Hinchliffe admitted causing the injury but pleaded not guilty to attempted murder, claiming he was acting in self-defence when Mr Williams came at him with a bottle.
The trial heard the pair had been drinking together for more than 30 hours before the incident.
During the trial, the jury heard evidence from Mr Williams that before his throat was slit, Hinchliffe fired a gun at him and the bullet had grazed the back of his head.
But Hinchliffe claimed in his defence that Mr Williams had asked to borrow his gun after arranging to buy drugs from a friend and the weapon accidentally discharged when he hit Mr Williams on the back of the head and said "you're an f-ing idiot". The firing of the gun was not part of the attempted murder charge.
Mr Williams had claimed he and Mr Hinchliffe had spent their evening trawling bars in Rockingham looking for bikies to give a "bashing" because Hinchliffe "wanted to settle the score" after speaking to the president of an outlaw motorcycle gang who called him a "piece of sh-t".
Mr Williams said when they were unable to find any bikies, they went back to Hinchliffe's house but things between them were "edgy" because he had not stayed in the car as he was asked to.
Mr Williams claimed he was punched and hit with a bottle before Hinchliffe fired the gun and cut his throat.
The defence case argued the pair had not gone out looking for bikies, but were rather searching for a love interest of Hinchliffe's and the argument was sparked by a work-related issue and Mr Williams wanting to buy drugs.
In his evidence, Hinchliffe claimed it was a "complete accident" that Mr Williams was injured and he grabbed the knife believing his workmate was going to attack him with a beer bottle.
Closing the State's case yesterday, prosecutor Laura Christian argued Hinchliffe had deliberately tailored his evidence and his account of how the gun went off was implausible.
She also suggested that Hinchliffe's actions after Mr Williams was injured, including getting rid of the gun and hiding the knife, were consistent with him knowing he was guilty of trying to kill Mr Williams.
Defence lawyer Laurie Levy dismissed the prosecution's argument as "nonsense" and he argued Mr Williams had wavered in his account of what happened.
He said Hinchliffe had armed himself with the knife in self-defence and he would have used the fully-loaded gun, rather than kitchen knife, if he intended to kill Mr Williams.
Hinchliffe will be sentenced in April.
He was remanded in custody.