Mike --
Thanks for the information, re: your SO5, Browning and Baker. This "4 inch, maximum differential," metric seems consistent with some information I remember reading, years ago. (I cannot remember where.) Its seems that this was considered a reasonable manufacturing tolerance for double guns. But, if you do the arithmetic, assuming straight barrels, this 4 inch point of impact tolerance implies an 0.078 inch difference in spacing of the bore axes - front to back - along the length of 28 inch long barrels. That is a very large value for a manufacturing tolerance for this dimension. Perhaps the assumption of straight barrels is a poor one. Additionally, there is the question of the axial accuracy of threading of the barrels for the chokes. It all adds up. Interesting -- if disconcerting.
Best Regards,
Don
