Albanian national Klejdi Kazani was under investigation by the Organised Crime Partnership (OCP) when he was arrested in August last year.
Investigators from the group – a National Crime Agency and Metropolitan Police Service unit – had tracked him travelling on the M1.
They tracked him on the route from Birmingham to London and shared intelligence with West Midlands Police who moved in to stop the car.
It was brought to a stop in Hertfordshire, where officers recovered a converted blank-firing Turkish Retay 84FS self-loading pistol hidden under the front passenger seat. The weapon was able to fire live ammunition as its barrel blockage had been removed.
The 21-year-old, of no fixed abode, was sitting in the back and attempted to flee, at which point a cigarette packet fell from his trouser pocket. He was arrested and several rounds of Italian Fiocchi 9mm blank cartridges, which had been modified by adding a steel ball bearing as a projectile, were found in the packet.
The driver of the car was also arrested and remains under investigation by the OCP.
Kazani pleaded guilty to possession of a firearm with intent to endanger life, possession of ammunition with intent to endanger life and possession of a prohibited firearm, at Luton Crown Court on May 12.
He was jailed at the same court on May 18 for seven years and six months for the firearm charge, seven years and six months to run concurrently for the ammunition charge, and six years to run concurrently for the prohibited firearm charge.
Andrew Tickner, from the Organised Crime Partnership, said: “Kazani was caught with a deadly weapon and ammunition, which had the potential to inflict serious violence and fear on our streets.
“I’d like to thank West Midlands Police for their assistance in arresting Kazani.
“We will continue do all we can to protect the public from the threat of firearms, and pursue the organised criminals who supply them.”