A fleeing criminal aimed a gun at the face of a man and pulled the trigger as he tried to escape from the police.
The gun failed to fire and the wanted man was captured in West Hampstead, north-west London on Tuesday.
He had turned the pistol on the man after trying to steal his moped in his attempt to evade officers.
Police had stopped a grey Vauxhall Astra in West End Lane, before the suspect and a second man ran off after pointing the gun at officers.
The officers gave chase as the suspect crossed a railway line near West Hampstead tube station and attempted to steal the moped in Sheriff Road.
The suspect was eventually arrested after he was found hiding beneath a car by armed officers a short distance away in the Hemstal Road area.
The second man was held a short distance from West Hampstead tube station.
Three guns and a quantity of diamonds wrapped in paper were recovered.
Police said the moped rider was unhurt but had been left deeply shocked and is receiving support.
Ballistics experts are carrying out tests to find out why the gun did not go off.
Det Insp Mick Millar said: "This was a serious incident in which officers and a member of the public were threatened with firearms in broad daylight in a very busy area of north London.
"This incident could easily have resulted in somebody being shot and seriously injured if not killed."
Two men, aged 30 and 47, are being questioned by detectives at a north London police station.