Sir Ralph has one more salient point to add to this topic:
Let me here remark that, for all-round shooting, there is no advantage whatever in the traditional custom of having one barrel of a gun bored to fire a closer pattern than the other, especially as regards driven game, as in the latter case, taking the shots that offer one with another, there is no constant difference in the ranges at which the birds pass overhead; and, even when walking up partridges or grouse, it generally occurs that the first to rise are the old birds, as they are the wildest, and hence a long shot to the right barrel is as likely as to the left one. Gunmakers seem to imagine that it is the constant habit of game to rise in pairs, and offer a near and then a distant shot in succession to each barrel as fired. If a shooter fired all his life at pigeons sprung from traps, or made it a rule to miss with his first barrel, it would be another matter.
THE SELECTION OF A GUN IN REGARD TO ITS WEIGHT AND CHARGE, By Sir Ralph Payne-Gallwey
As a side note, he also concludes that subgauges are to be avoided:
Guns of 16-, 20-, and 28-bore always handle and feel pleasant, especially in a gunmaker's shop, and give the idea that they must be easy to hit with as a natural consequence; but from numerous experiments, both at the target and in the field, I can positively state that any bore smaller than a 12 is not so effective on game, nor so easy to aim straight with, as the latter size. Small-gauge barrels of necessity shoot weaker than a larger size: they have a more open pattern if cylinders, and a closer if chokes; they do not shoot so regularly, and, in proportion, recoil more than do 12-bores.*
Perhaps this explains why early Brit subgauges are so scarce