Antique ammunition was found in a safe during the clearance of the former ColArt art materials factory in Wealdstone.
The factory in Whitefriars Avenue, which opened in 1937 and produced Windsor and Newton brand coloured paints and tools, has been closed and its work transferred to a production facility in Le Mans in France with the loss of 190 jobs.
The Metropolitan Police's firearms unit was called out to decommission the live old bullets discovered on Tuesday.
It has been sent off for analysis but is believed to be ammunition for a Second World War firearm.
Harrow borough commander Chief Superintendent Dal Babu attended the call along with two work experience candidates from Nower Hill High School sixth form that he was mentoring.