An Adelaide judge has called for all guns to be banned during a court hearing over the fatal shooting of a teenager.

Charles Cullen may not have pulled the trigger, but Judge Paul Muscat believes that if it were not for him, Lewis McPherson could still be alive.

Cullen sold a pistol for $900 to teen killer Liam Humbles, who then used it to murder his schoolmate on New Year's Eve in 2012.

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Judge Muscat was so concerned by the fatal chain of events, he used today's hearing to call for a total ban on firearms.

“Guns should never be possessed because of the inherent dangers, and this case was a stark example,” he said.

His call was backed by Lewis's father.

“You have to give judge Muscat credit - absolute credit - for his words and the way he's approaching this case,” Mark McPherson said.

As Cullen faced the District court to plea for a lenient sentence, Judge Muscat warned him to expect anything but.

Despite that, Cullen offered an apology to Lewis's shattered family.

"Although I did not pull the trigger that night, I still feel incredibly guilt ridden," he said.

"I ask you to remember that I was not the man that took your son's life."

Charles Cullen will be sentenced next week.