Twenty four guns and thousands of rounds of ammunition have been seized in County Down in a joint operation involving police, National Crime Agency (NCA) and Border Force officers.
The weapons were found when officers raided a house in Ardkeen, County Down, on Friday morning.
A 61-year-old man was arrested and later released "under investigation".
The raid came after a semi-automatic rifle and two hand guns were intercepted at a parcel hub in England.
Border Force officers discovered the AR-15 rifle and hand guns at the premises in the Midlands earlier this month.
The man who was arrested on Friday is a registered firearms holder.
The National Crime Agency (NCA) described the weapons haul as huge but it is not believed to be linked to paramilitaries.
"This operation is an excellent example of the cooperation between the NCA, PSNI and our Border Force colleagues," said NCA branch commander David Cunningham.
"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."