I think that the "used to be commonly known" fact that double shotgun barrels are not soldered together straight has been misplaced by modern shotgunners. Also, "shotgun pattern convergence" is a contradiction in terms. Two barrels whose centers are, let's say .410 inches apart, do not converge at 40 yards. They both shoot a straight line to the target. Identifying a .410 inch difference in point of impact in a 40 yard pattern is beyond the capability of any human.