Specialist officers arrested four men in the early hours of Thursday morning after finding a gun during an "intelligence-led operation" in Hampstead Garden Suburb.
Scotland Yard said a firearm was found when two cars were stopped in Addison Way at around 12.30am on February 18.
Firearms officers were involved in the stop and the two cars were searched, the Met said.
Four men, aged 23, 24 and 46, have been arrested on suspicion of possession of a firearm. The men remain in police custody.
Det Insp Glenn Butler, from the Met's Specialist Crime Command, said: "Due to the dedicated and hard work of officers from the Specialist Crime Command we have been able to successfully seize and remove a lethal weapon from the streets of London.
"Being in possession of a weapon such as this is unacceptable and poses an abhorrent risk to people's lives. We will continue to do all that we can to keep people and communities safe."
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