The find was made during what police described as a “proactive” search of a house in the Crumlin Road area yesterday morning.
A 50-year-old woman and 16-year-old boy were arrested on suspicion of possession of a firearm in suspicious circumstances.
They were quizzed at Musgrave PSNI station and then released later last night on police bail pending further enquiries.
A 57-year-old man was questioned by police after being arrested this morning on suspicion of a firearm and ammunition in suspicious circumstances.
He has also since been released on bail pending further enquiries.