Hello,

One of the other factors in the theory, is that the CG of the barrels falls in the center of the rib.
The R barrel recoils around the CG which is to the L of the barrel, while the L barrel recoils around the CG that is to the R of the barrel.
Depending of rifeling which may be R handed and L handed per barrel or both R handed, and you see the complexity of the problem.

Cheers
Nitro

PS: Mac knows his stuff.