There is also the effect of recoil that moves the movement laterally and slightly down. If you have access to other barrel sets you can measure the center of the breach and the center of the muzzles and find the basic angles. When relaying ribs I use a jig that has level surfaces to lay the barrels on so the center of the bores are parallel at the muzzle. That leaves the patterns vertically, and the angles of convergence will overlap the patterns horizontally. The rub comes in the fact if you just bore sight the barrels you may not get where you want. They may appear to crossfire sooner than you want .
I only have two other doubles. One is an OU and the first barrel tends to shoot a little low. The second shoots dead on. Both are centered. The other is a .410 side by side. One barrel shoots a little low left. Second barrel shoots dead on. This is based on fixed patterning with a sheet of butcher paper stretched between two posts. If I adjust I can hit with them in the field.
