A 61-year-old man has been arrested after police seized 25 firearms -including a semi-automatic rifle - which were intercepted in the post.
The weapons and thousands of rounds of ammunition were recovered in a joint operation between the National Crime Agency and Police Service of Northern Ireland.
The investigation was launched after an AR-15 semi-automatic rifle and two handguns were intercepted by Border Force officers at an international parcel hub in the Midlands earlier in November.
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 lives 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.”