Vincent Kelly, a 31-year-old Belfast man, was caught with the firearm and ammunition almost two years ago.
He was sentenced by Belfast Crown Court, sitting in Antrim, this morning.
He had pleaded guilty to possession of a firearm with intent to endanger life, and was sentenced to nine years imprisonment.
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Kelly, formerly of Hawthorn Street in west Belfast, got off a bus from Dublin in Belfast city centre and was travelling in a taxi along Grosvenor Road when he was stopped by police on November 20, 2014.
The PSNI said that inside his black sports bag officers found a reactivated and modified PPS43 submachine gun, three magazines and 50 rounds of ammunition.
When the weapon was forensically examined and test fired, it was discovered that the submachine gun could only be fired on fully automatic mode, “thereby increasing its lethal capability”, according to the police.
Detective Chief Inspector Gillian Kearney said: “We are delighted to have taken this lethal weapon off the streets.
“Lives have undoubtedly been saved due to the work of officers in Serious Crime Branch, supported by our colleagues in An Garda Siochana.
“Those who follow a path of violence need to realise that their activities will inevitably lead to serious consequences for themselves.
“Police remain committed to taking illegal firearms out of circulation and putting those involved in this potentially deadly activity before the courts and behind bars.”