It ended with 22 shotgun pellets stuck in the chest and left arm of a good Samaritan, most likely for the rest of his life.

And now the startling events of May 30 last year will cost gunman Benjamin Darrell Hallam his freedom for at least two years and five months.

Justice John Burns imposed that non-parole period in the ACT Supreme Court on Friday as he sentenced the 33-year-old shooter to a total jail term of four years and five months.

The judge told Hallam he had to deter other would-be vigilantes from taking the law into their own hands, noting in this case the misguided gunman had seriously injured an innocent grandfather.

In sentencing, Justice Burns said Hallam had been drinking with people including his on-and-off partner Isabella Denis and best friend Paul Fredrickson at a party in Dunlop on the day in question.

The 33-year-old argued with Denis, prompting the latter to storm off in search of a nearby friend's house.

Denis accidentally went to a place three doors down from her intended destination, before realising her mistake and wandering away to lie down on the road.

When the well-meaning couple she had disturbed tried to coax her to safety, she punched the male good Samaritan in the face and kicked over a letterbox.

Denis then phoned Hallam and lied to him about being "bashed by five black guys".

An incensed Hallam therefore took a shotgun and Fredrickson, who was armed with a blue baseball bat, to the victim's home.

Fredrickson smashed a window at the front of the house, then tussled through the broken glass with the good Samaritan.

While the pair were struggling over the bat, Hallam fired a single shot through the front door from about two-and-a-half metres away, hitting the victim in the chest.

Hallam and Fredrickson then fled, dumping the gun and bat nearby.

But Hallam left his Toyota Landcruiser, which contained ammunition of the sort used in the shooting, parked not far from the victim's house.

He was subsequently arrested at his Charnwood home, where investigators located six cannabis plants growing in a bedroom.

Searchers also spotted the baseball bat floating in a pond near the scene, before Fredrickson led police to a second pond where Hallam had stashed the shotgun.

Hallam ultimately pleaded guilty to charges of recklessly inflicting grievous bodily harm, discharging a firearm at a building, using a prohibited firearm, attempting to conceal evidence, and cultivating cannabis.

On Friday, Justice Burns said he accepted Hallam's evidence the 33-year-old did not deliberately shoot the victim.

Hallam had previously told the court he had tried to fire his weapon into the air to scare whoever had apparently assaulted Denis, but the gun did not go off until he lowered it down by his side and it discharged into the front door.

Justice Burns said this did not absolve Hallam of responsibility, saying the 33-year-old had behaved in a "highly reckless" way instead of doing the sensible thing and reporting the bashing allegation to the police.

The judge also said Hallam had not acted spontaneously, having had time to consider what he was doing en route to the victim's home.

"Your moral culpability ... is high," Justice Burns told Hallam.

The judge went on to accept Hallam's expressions of remorse as genuine, adding he did not think the 33-year-old would reoffend in a similar way.

But he said he still had to "mark the community's disapproval" of circumstances where an innocent person had been seriously injured.

"I must impose a sentence that brings home to those who may be minded to take the law into their own hands that their actions will have serious consequences," Justice Burns said.

Hallam's sentence was backdated by 16 days to reflect the time he spent in custody following his arrest.

He will therefore become eligible for parole in November 2023.

A number of Hallam's supporters called out "love you" from the courtroom's public gallery as he was led away by officers from ACT Corrective Services.

Fredrickson was jailed last year for his role in the relevant events, and will be eligible for parole this month.

Denis, meanwhile, received a good behaviour order.

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