A 77-year-old pensioner was arrested after police stopped two cars in south London and found a loaded assault rifle.
The Armi-Jager AP80 rifle, with a loaded magazine, was discovered in the boot of one of the cars, which was being driven by a 50-year-old man.
Detectives believe the gun is fully automatic and would have been used to kill or seriously injure someone if it had not been seized.
Police said the second vehicle was being driven by the older man.
Both men were detained by officers after they pulled the cars over in Peckham on Thursday evening.
Scotland Yard said the 50-year-old was tasered by armed officers after he resisted arrest. He was treated for minor injuries in hospital and later taken into custody.
The men were both arrested on suspicion of possession of a firearm and ammunition with intent to endanger life.
“I have no doubt that removing this weapon from circulation has prevented death or serious injury," said Detective Chief Inspector James Harper.
He added: “Officers will continue to work tirelessly to disrupt and detect such criminality.”
