From the article I am guessing a slightly downward bent barrel would make it tend to loft upward.
It has been previously reported that a recent development in paint ball guns is a long barreled gun with an intentionally downward bent barrel to cause the aforementioned spin to increase range and accuracy.
An interesting experiment would be to mount the trunnions off center below the bore centerline with a resilient link between the breech and elevation screw to intentionally cause the barrel to flip the muzzle when firing so that the shot would be caused to roll against the bottom of the bore and get a top spin to cause lift. The vent should be drilled through the cascable so the barrel could be inverted and the experiment tried with the trunnions above center, too, since the dynamic nature of the barrel flip might actually give the opposite of the most obvious effect. You could alternatively put the trunnions on a band and rotate the barrel in the band to reposition the curve.