Reason I asked is that I find it absurd that the trigger selection would have any significant and negative effect on regulation when one of the crowning glories of a double rifle is the ability to have instant access to two different bullet types; solid or soft. That trigger selection would have some deleterious effect on the grouping of the rifle would be to scratch a significant advantage of the double.

That left-shoulder shooting might have some impact on regulation I would not find surprising, as we do essentially all our shooting from field positions {with bolt guns} and find that groups shot from unsupported field positions do not always have a POI identical to the POI of the same rifle and load shot from the bench. If the zero of a bolt gun can shift due to hold, I suspect the zero of a double can be effected by the shoulder from which it is shot though I am not certain about that as I have no experience to prove it to me.