As far as im aware of, there is 2 shotguns, the one barreled one and the other one. The one barreled one is the one which you get when you kill the shop assistant in Ammunition and the other is on top of this construction part which is located in that private building with the gate, the one near when you take a picture of Casa in that baby suit.
If this doesnt help then my answer to his good shotgun is no and in order to get it would require cheats.
Hope i helped
Bloody Baron
GTA Genius
Eminem: The Legend of Legends