William Adair Rothwell, 18, and Jacob Michael Brennan, 17, carried out the murder in August last year for a "trivial and dishonourable reason" and received half an ounce of cannabis each as payment.

It was one of Tasmania's longest murder sentences since Marco Daniel Rusterholz received 45 years for two murders in Ravenswood.

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Justice Robert Pearce took the extraordinary step of allowing the youthful killers to be identified in the interests of justice and to highlight heinous nature of the crime.

There was an audible gasp in the courtroom when the sentences were handed down in the Launceston Supreme Court.

Billy Ray Waters' grandmother Denise Waters said justice was done but admitted she was shocked by the sentence length.

"Just to hear it, gosh," she said amid tears.

"I didn't think that these boys were going to lose so much of their life for what they done.

"It's unbelievable, all for a bit of weed.

Justice Pearce, who ordered a non-parole period of 15 years, warned the Parole Board that it needed to think carefully about granting parole, particularly to Brennan, who stabbed Billy Ray 18 times.

"Given the uncontrolled and unrestrained nature of the attack there could be future risk to the community upon release," he said.

"Before Brennan is released the Parole Board needs to carry out some psychiatric assessment on the level of danger that he poses."

Rothwell encouraged Brennan to be involved and took the shotgun and a Bowie knife and a baton to bushland in Mowbray where they lured Billy Ray.

Brennan shot him in the leg and when they retreated Billy Ray ran after them calling for help.

"When he pleaded for help you acted without mercy," Justice Pearce said.

He was stabbed and hit over the head with a baton before one said "he's not done" and fired the fatal shot into the back of his head.