Jason Pikula-Carroll, 26, pleaded guilty to six charges, including murder by joint commission, following two home invasions at the Watson unit of artist and small-time drug dealer Eden Waugh.

Pikula-Carroll had targeted the victim after a heroin deal went awry, with sentencing judge Justice John Burns describing him as "the driving force" behind the first incident in September 2016.

On that occasion, Pikula-Carroll sought retribution for the failed drug transaction by taking a shotgun and armed co-offenders Peter Forster-Jones and Phouthakone Sikounnabouth to Mr Waugh's unit.

The trio broke in and demanded drugs and cash, unleashing carnage that injured Mr Waugh and others inside the home.

Six weeks later, concerned Mr Waugh could identify them as police investigated the incident, the offenders returned to the Watson unit in a bid to stop him co-operating with authorities.

This time, Sikounnabouth waited in a car as the getaway driver while Pikula-Carroll and Forster-Jones went up to the unit.

Upon realising that Mr Waugh was barricading the entrance, Forster-Jones fired the same shotgun from the earlier incident through the front door and killed him.

Pikula-Carroll then dragged Mr Waugh's partner, Marion Barr, across the floor by the hair as he and Forster-Jones went inside and pillaged the place of valuables.

Among the things they stole was Ms Barr's phone, which was, unbeknownst to the offenders, connected to a triple zero call as they fled.

In a recording of the call, Pikula-Carroll could be heard "cheering and yahooing", as prosecutor Anthony Williamson put it, in praise of Forster-Jones.

Justice Burns ultimately sentenced Pikula-Carroll to the same jail term imposed on Forster-Jones, saying the pair were similarly culpable because, while the latter had fired the fatal shot, the former had sourced the gun and set the relevant events in motion.

Both killers were jailed for 30 years, with a non-parole period of 18 years.

Pikula-Carroll challenged the severity of his sentence, on grounds including that it was "manifestly excessive", in the ACT Court of Appeal last November.

Arguments raised by his barrister, John Purnell SC, included that Justice Burns had been wrong to find Pikula-Carroll had been aware the shotgun was loaded before Forster-Jones fired it.

Three appeal judges - Justice David Mossop, Justice Katrina Banks-Smith and Acting Justice Verity McWilliam - agreed with Mr Purnell on this issue in a judgment handed down on Friday.

The trio said an alternative hypothesis - that Pikula-Carroll merely knew it was possible the gun was loaded - had not been excluded beyond reasonable doubt.

The appeal judges therefore needed to resentence the 26-year-old, who faced court via audio-visual link from prison on Friday as Justice Mossop pronounced his new jail term.

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Pikula-Carroll's sentence, backdated to start in March 2019 to account for time served, was reduced from 30 years behind bars to 27.

Some 22 months were also shaved off his non-parole period, which is now 16 years and two months.

Pikula-Carroll will therefore become eligible for release in May 2035.

Forster-Jones, who also admitted murder and began serving his sentence earlier, will become eligible for parole in February 2036.

Sikounnabouth will be in custody, meanwhile, until at least November 2026. He pleaded guilty to charges that included being an accessory to murder and was jailed for 12 years, with a non-parole period of seven years and three months.

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