"Boys being boys, but look at the outcome. Put yourself in our situation. We've lost our son and while it was an accident, they should never have been in possession of that weapon," he said on Thursday after a coroner found Daniel Cowan died from being hit in the face by a "high velocity missile".

That missile was a bicycle handle grip wrapped in electrical tape and fired from a gun powered by compressed air, a combination that caused a devastating head wound.

Daniel, 15, was hit while standing over the gun's 1.4 metre long PVC pipe barrel when a friend, Mitchell Seeley, pulled the firing lever in the belief no one was in the way.

Daniel, his twin brother Thomas and another boy were at Mitchell's home in Heathcote Junction, north of Melbourne, when he was hit on the afternoon of October 2, 2012. He died eight days later in the Royal Children's Hospital.

The boys had been firing the gun, which was connected to an compressed air hose, without adult supervision although Mitchell's mother, Vicki, was inside the family house at the time, the Coroners Court heard.

Mitchell's father, Mark Seeley, had been given the air gun, commonly referred to as a spud gun, by a colleague a week earlier and had modified it himself, coroner John Olle said.

The night before Daniel was hit, the court heard, Seeley and his son had fired the gun about 10 times while wearing safety glasses and ear muffs, before Mitchell was told not to use it without his father being present.

Mr Olle said the air gun was so powerful it was later defined as a firearm by police, and hoped this "absolute tragedy" would highlight the dangers posed by homemade weapons.

Twenty people had died from unintentional gunshot wounds in Victoria between July 2000 and December last year, he said.

Outside court, Mr Cowan said he and wife Lisa understood their son's death was an accident, but were angry the boys could get their hands on the gun, which had been in the Seeley family's shed.

"These homemade air guns are a lethal weapon and are no different to giving a kid a shotgun to play with. It beggars belief that the boys had access to this lethal weapon to play with on the day," Mr Cowan said.

He recalled a surgeon saying Daniel's injuries were similar to the type inflicted by shrapnel in a war zone.

He and his wife were determined to raise awareness of the dangers to spare another family similar tragedy.

"I hope no one has to experience what we've had to experience," he said.

Seeley did not have a firearms licence at the time, the court heard. He pleaded guilty to one charge of possessing an unregistered longarm in February.