The items were found in Drumgask Road on Friday morning by a member of the public, who then alerted police.
A police spokeswoman said: "While the items appeared to have been there for some time, the weapon was loaded and presented a clear danger."
She said officers were "grateful" to the person who reported the gun so it could be disposed of safely.
Police have seized a sawn-off shotgun and a quantity of ammunition in Craigavon, County Armagh.