A police station in Cheltenham had to be evacuated after a live grenade and a quantity of ammunition were handed in.
The finder discovered the items in a field using a metal detector.
A force spokeswoman said anyone finding such items should leave them, move a safe distance away and alert the emergency services.
"Old explosives can be very unstable and so for everyone's safety such devices should be left where they are," she said.
The police station was evacuated for a short time and the items were disposed of by a bomb disposal unit.
It is not the first time explosives had been handed in to the Lansdown Road station by members of the public.
In June 2008 a World War II grenade was brought into the station with the pin still in after being found at an address in Longlevens.
It was handed over to officers wrapped in cloth inside a shoe.