Smoothbores can be a finicky breed. It may take some time to fine what it likes. Size and amount of powder, size of ball, thickness of patch. I have one smoothie that shoots like a rifle out to fifty yards. The shadow of the tang screw is my rear sight. I just line it up with the front bead, not seeing any of the barrel and she shoots center. Guns like that are rare...I know because I've had a few smooth bores over the years.
I wouldn't put a rear sight on the gun. Be patient, and work with it, from what I see it looks like you've got a good one.