Two Scorpion machine guns and 40 rounds of ammunition have been seized after Met officers arrested a 19-year-old man near Chiswick Park Tube station.
The man was arrested earlier on suspicion of possessing a firearm with intent to endanger life.
He was taken to a west London police station.
Det Ch Supt Craig Turner said: "[The guns] could have undoubtedly led to the loss of life had they not been recovered."
He added: "Anyone who believes they can walk the streets with horrific weapons in their possession should think again."
The arrest was part of an operation by the Met's Specialist Crime Command and Specialist Firearms Command.