Justice Patrick Keane and Justice James Edelman threw out David Allen Will's bid to have his jail term slashed on Friday afternoon, prompting tears from a woman in the court's public gallery.

Will, 66, was the brains behind the May 2004 robbery of an armoured van outside the Mawson Club, conceiving the plan and paying a Chubb employee $15,000 for inside information about the vehicle's movements.

He recruited Mark Anthony Munro and Sam John Melkie to perform the daylight heist, and the pair wielded guns when they confronted some van guards during an afternoon cash pick-up.

Munro blasted a guard, who survived, with a sawn-off shotgun at close range, spraying pellets across the victim's face, left arm, abdomen and chest.

The robbers ultimately made off with a total of $151,995 in cash.

Many years later, Melkie pleaded guilty to aggravated robbery and was sentenced to nine years in jail.

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Munro, however, pleaded not guilty to the same charge and to an allegation of intentionally inflicting grievous bodily harm.

He stood trial and was found guilty, then locked up for 15 years, after Will gave evidence against him.

It was not until after the cases of Munro and Melkie had concluded that Will himself was charged, and eventually convicted of aiding, abetting, counselling or procuring an aggravated robbery.

He was sentenced by Justice David Mossop in 2018 to 10 years and 10 months behind bars, with a non-parole period of six years.

Will challenged the severity of that term in the ACT Court of Appeal earlier this year, arguing he should have received a discount for his assistance in the prosecution of Munro.

A 2-1 majority of that court dismissed his appeal, finding he would have needed to give that assistance voluntarily, or willingly, to qualify for such a discount. Will had testified under subpoena.

The dissenting judge, Justice Chrissa Loukas-Karlsson, believed there had been a miscarriage of justice and Will should be resentenced, with a discount applied.

On Friday, during an application for special leave to appeal to the High Court, Will's barrister, Kieran Ginges, argued this interpretation was correct and the other two judges, Chief Justice Helen Murrell and Justice Natalie Charlesworth, had been wrong to conclude otherwise.

Mr Ginges urged the High Court to resolve the Court of Appeal judges' differences of opinion, saying an aspect of the ACT's criminal law would otherwise be "left in limbo".

He also said his client had given truthful evidence that had resulted in the conviction of Munro.

Will should have been rewarded for this, Mr Ginges argued, because it would be "naive" to think everyone who testified in court told the truth even when they were legally required to.

But Justice Keane questioned why giving truthful evidence should be considered anything other than "doing your duty like any other citizen".

After briefly adjourning to confer with Justice Edelman, he dismissed the application.

"This appeal does not enjoy sufficient prospects of success," Justice Keane said, without needing to hear from prosecutor Kylie Weston-Scheuber.

Will's non-parole period expires in May 2024.

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