Old sub-machineguns are rare in Australia but will begin surfacing more often as their elderly owners die, according to a specialist firearms lawyer.

Police say they seized a Thompson sub-machinegun from a man's bag in the Marrickville Metro shopping centre carpark in Sydney's inner-west on Wednesday. Marrickville resident Mark Avidov, 41, was charged with prohibited firearm and ammunition offences.

Gun law expert Simon Munslow said the Thompson was "rare and in fact expensive". However, "quite a lot" were brought to Australia from Europe by returned servicemen at the end of the World War II.

"You are going to start seeing a few of them percolating up because they have been in old house's attics," Mr Munslow said. "The old fellow dies and the gun shows up."

Mr Munslow said Thompsons belonged to Australia's large grey market of once-legal guns but they were not popular.

Organised criminal syndicates tend to prefer hand guns because they were more concealable whereas terrorists often opt for AK-47s.

He said an amateur using a Thompson would most likely send a rapid burst of bullets upward as they struggled with unexpected recoil.

"It takes a lot of skill and training for someone to work the trigger."

Mr Avidov, if convicted, faces a maximum 14 years' jail for possessing a prohibited gun.

"You're going to be fighting a battle to keep someone out of jail if they possess that sort of firearm," Mr Munslow said.