Chuck --
I believe your point about fixing the barrels in position and sighting through the barrels at a distant mark to assess the alignment of the barrels is correct and a good suggestion. A mark centered in the view presented in one barrel should also appear centered in the other barrel -- with correct / parallel barrel alignment. But, as you probably know, one must have the eye precisely located along the axes of the bores in making this evaluation. When aligned with the axis of a straight barrel the eye sees a set of concentric rings. Concentric rings will not be found, if the barrel is not straight. Some ring distortions will be found in this case.
I undertook this procedure, when I purchased the gun and the result looked reasonably good. I could not ascertain any significant difficulties / misalignment. Even the ring concentricity appeared reasonably good.
As impied in your note, all together -- these findings may indicate that the threading for the chokes is at fault. But, the choke threading appears to be centered in the bore of the barrels at the muzzle. The wall thickness appears to be uniform around each barrels periphery. But, I can more carefully measure these wall thicknesses as you suggest. If the threading for the chokes, alone, was the cause of the differential impact and if the difference was ascribed to the threading of one barrel, choke threading would have to be "off by approximately 0.021 inches to explain the result. That is a lot of choke misalignment. Perhaps both chokes are "off" -- but opposed directions in the same plane.
Of course all the tolerances add up ... each barrel's straighness, brazing alignment, choke threading, etc. It would take some very careful measurements to "sort this all out". I am not equipped to make these measurements. And, right now, I am a little discouraged by the situation. But, in the end I will have a grouse gun that "shoots straight"..... "come hell or high water"..... Rizzini notwithstanding.
Thanks again.
Best Regards,
Don
