When he was 13 years old, Paul Munro went with his grandfather, who fought in Gallipoli, to buy his first gun, a .22 rifle.

Fifty years later he's awaiting sentencing for importing high-powered automatic rifles from the US, one of which was used in a $280,000 armoured van robbery in Victoria.

Munro faced a plea hearing on Friday in the Victorian County Court after admitting to importing six automatic rifles and 30 semi-automatic pistols, and attempting to import even more.

The 63-year-old was a legitimate gun dealer until his licence was revoked in 2012 for a misdemeanour.

He was arrested years later after police seized an automatic rifle used to rob $280,000 from a cash-in-transit van in Sunbury in 2014.

The gun was made by US company Thureon Defense, and was not available to buy in Australia.

A number of other Thureon rifles were seized around the same period, and police were determined to track down the source.

Victoria Police teamed up with America's Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) in an investigation that culminated in Munro's 2016 arrest.

The court heard an undercover officer contacted Munro, who lived in Koraleigh on the NSW border, about buying guns.

They met at a McDonald's in Bendigo, where Munro showed the undercover cop a sample gun.

The covert officer agreed to buy 15 guns for $110,000 before police swooped in and made the arrest.

Commonwealth prosecutor Linda Skoblar said the Thureon rifles could fire 1000 rounds per minute.

As a way to demonstrate their firepower, a video was played to the court, showing an officer in ear muffs blasting rounds in a shooting range.

"This will fire continuously until pressure is released from the trigger," the gun-wielding officer said.

Munro appeared to fall asleep in the dock several times during the hearing, prompting a clerk to alert Judge Douglas Trapnell.

Defence lawyer Brad Newton urged the judge to consider Munro's "deplorable state of health" when sentencing him.

He said Munro weighs about 186kg, had high blood pressure and had badly swollen legs and feet.

Judge Trapnell ordered a sentencing report for Munro and adjourned the case until September 15.