24 guns and thousands of rounds of ammunition have been seized following the search of a house in Newtownards.
A 61-year-old man who lived at the property in Ardkeen and is a registered firearms holder was arrested following the joint operation carried out by the PSNI and the National Crime Agency on Friday.
He has now been released.
The investigation was mounted after an AR-15 semi-automatic rifle and two handguns were discovered by Border Force officers at an international parcel hub in the midlands.
NCA Branch Commander David Cunningham said: "This operation is an excellent example of the cooperation between the NCA, PSNI and our Border Force colleagues. Together we have identified and recovered a significant quantity of illegal firearms and ammunition that could have been used to commit serious violence or other criminality."
There is no suggestion that there are any paramilitary links in the investigation.