Twenty-four guns and thousands of rounds of ammunition have been seized from a house in Newtownards.
The property in Ardkeen was raided on Friday, in a joint operation by the National Crime Agency, the PSNI, and Border forces.
The 61 year old man who lives there is a registered firearms holder.
He was arrested and released under investigation.
In a statement the NCA said:
"Twenty-four firearms and thousands of rounds of ammunition have been seized in a joint operation between the National Crime Agency and the Police Service of Northern Ireland."
"The investigation commenced following the interception of an AR-15 semi-automatic rifle and two hand guns by Border Force officers at an international parcel hub in the Midlands earlier this month."
"Officers recovered the firearms and ammunition haul during a raid at a house in Ardkeen, Newtownards, on Friday morning".
"The 61 year old man, who lived at the property and is a registered firearms holder, was arrested and released under investigation."
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.”