Two shotguns have been found over the weekend - one in a graveyard and another in a hedge.
Police in Belfast said the first weapon was discovered by members of the public in a part of Dundonald Cemetery on Saturday afternoon.
A PSNI spokesman said the sawn-off shotgun would undergo forensic analysis.
"There is now an investigation ongoing as we seek to establish where the weapon came from and for how long it had been there," the officer said.
Meanwhile, police have arrested three men aged 20, 26 and 41 on suspicion of possession of a firearm in suspicious circumstances.
The arrests followed the unrelated discovery of the second shotgun on the Castlewellan Road in Newcastle in the early hours of yesterday.
Police are investigating a link to a burglary at a nearby property.
Inspector Lynsey Barr said: "A passing police patrol noticed the men acting suspiciously at around 2am on Sunday, and after an examination of the area where they had been standing a shotgun was discovered, concealed in a hedge."