The discussion on scoping a double rifle and the suggestion of perhaps using a lightweight red-dot sight has brought a question to mind which I think could start an interesting and helpful new discussion.

We all accept that scoping a double can have an effect on regulation, right?
What do those in the know think is happening here? My guess is an alteration of harmonics is the culprit.
How makers can regulate a double for open and scoped use is a mystery to me, but it has been and is done and could be a separate discussion of its own.

Now, to my question at hand.
My feeling is that a superlight red-dot sight mounted near the breech would have no discernable effect on regulation.
My reasoning lies in the assumption that a bigbore double would be regulated shooting right/left. The left being fired with an empty shell in the right chamber.
Would the same rifle not regulate quite as well if shot left/right?
Would a sight weighing in at an ounce or two have any more effect on regulation as having one empty chamber?