Where a side-by-side will generally converge or diverge, an over-under will normally give elevation convergence or divergence.
; With experimentation of loads, one can sometimes bring the barrels together. Different projectiles, different wad columns and different powders have slightly different recoil impetus and this is what you use to get the loads to regulate. A 3-ring binder is not only usefull but necessary to log all development. Change one thing olny at a time. Cool stull really and as much or more work & certianly more finiky(sometimes) than working up a load for a competition rifle.
; The ultimate, of course, is to have group centres exactly bore axis apart at all ranges, neither converging nor diverging. This happened with my old fluid steel barrreled double, but was probably more luck than good management. It's oh, so rewarding to have developed such powerful and accurate ammo for your 'pelter'.