Police stumbled upon an ammunition “factory” in south London while searching for cannabis.
Officers unwittingly came across a cache of bullets and equipment for manufacturing ammunition after raiding an address in South Norwood on Wednesday.
They also discovered a handgun at the property in Fitzroy Gardens.
Police said they were only expecting to find cannabis during the early morning drugs raid.
Local officer Sgt Matiu Ratana tweeted: “We went looking for cannabis and found firearms.”
A spokesman for the Metropolitan Police said: “Officers recovered a quantity of various types of ammunition, equipment to make it, and a firearm from the location.”
No drugs were found at the property and no arrests have been made.
Scotland Yard said a probe was under way and confirmed the incident was not terrorism related.