I should have done a better job of making my point clearer, but I'm specifically referring to the experimental projectile seen in the photo, not the bolt with the real short wrought iron sabot that it evolved into. The sabot on that shot is four inches long, and I know it's somewhat corroded, but there is no sign of any rifling on it. I just don't think that that projectile would have ever been very accurate.
We've had discussions about parrott projos here before, both contemporary bolts (Norm's and yours), and historical. I don't at all question the validity of historic accounts of the Parrott's respectable accuracy, and that accuracy would have certainly depended on both the rifling of the gun and the construction of the bolt.