Hello, Huvius:
Certainly, the weight of the rifle and the barrels have a greater effect over regulation than harmonics, and I agree with you that the weight of my Leupold Deltapoint, a little over half an ounce will have a negligible effect over regulation. I think harmonics have a bigger role in the size of the groups of each barrel...
But There is another fact in regulation you mention, and it is the way the heat of the barrel you shoot first affects the other barrel. I do not know exactly the dynamics of heat trnasfer and dilatation and/or "deformation" of the second barrel. But I can tell you that in my 450 there is a clear effect: the rifle shoots different if you fire the left barrel first, regarding both regulation and point of impact.

Herrdoktor