During the attack Britton used a racial slur to describe his victim.
"The words were spoken in anger, although [the victim] did not understand why Mr Britton was angry with him," said Judge Carter.
When the punches stopped Britton told his victim: "You better f**k off now before I shoot you. I've got a gun in my bag, I'll f**king shoot your dog."
After grabbing his cellphone, the victim fled the scene but when he turned around to check where Britton was, he saw him taking his backpack off.
"He was scared for his life and scared for his dog's and that his kids were at home by themselves and he was worried that they wouldn't have a father," the judge said.
Britton was arrested and charged with injuring with intent to injure and threatening to kill.
On Thursday Judge Carter released his reserved decision, finding Britton guilty on both charges.
Defence lawyer Raukawa Simon said Britton, who was in custody, wanted to be sentenced on the charges as well as another of assault with intent to injure, which he pleaded guilty to last week.
On May 13, Britton had attacked a 70-year-old man at Whanganui's Splash Centre swimming complex.
Britton repeatedly hit the man with clenched fists around the head and torso, causing him to fall backwards into a swimming pool.
Simon sought a sentence of between 18 and 24 months' imprisonment.
Police prosecutor Sergeant Stephen Butler said the use of the racial slur was an aggravating feature and believed an end sentence of three years' jail was in order.
Judge Carter said the separate attacks had taken a toll on both victims.
The elderly man had suffered a cut to his bottom lip and injured shoulder in the attack, which had affected him emotionally and left him fearful for his safety and the safety of the public, Judge Carter said.
The Māori man had to take several weeks off work following the beating in which he suffered concussion, grazes and lost the vision in his left eye for a day.
Judge Carter said the victim was still experiencing ongoing anxiety and was fearful of another random attack so had stopped walking his dog along the riverbank.
He said it was an aggravating factor in the sentencing act if an offender committed the offence partly or wholly because of hostility towards a group of persons who have an enduring common characteristic such as race, colour, nationality, religion, gender identity, sexual orientation, age, or disability.
Britton was sentenced to a total of 22 months' imprisonment - 12 months for injuring with intent to injure, four months for threatening to kill, five months for assault with intent to injure and one month for the remission of $4997 in fines.