While I agree that "production" gun manufacturers do not provide absolute control on POI/convergence/etc., I think they do/did a fair job considering they never shot most of the guns at a board. Their downfall may be that some makers don't stand behind their product to the satisfaction of the customer.
My first BSS patterned with one barrel off a few inches (it's been 20 yrs since I patterned it) but I still found it acceptable to me.
To illustrate the precision or lack of precision required for achieving acceptable POI of both barrels, I can share my experience with the recent strip and re-laying of ribs on my Parker. I desoldered the ribs and the muzzles, cleaned between the barrels and all ribs, then wired them together with the muzzles touching as before, then leveled the barrels with the method shown in the picture. The barrels shoot nicely on top of eachother. I need to do more patterning to tell you by how much they misalign, but I did enough to see they obviously are well under what I consider maximum.
