The basic reference in shotgun design is the imaginary line of the bore axis. The breech is indeed at right angles to the bore axis. It is NOT 90 degrees to the water table, by definition it cannot be. Since the barrel breech ends are conical it follows that the water table and breech which fit the barrels form an angle smaller than 90 degrees.
Amazing how many experienced gunsmiths assume that the breech-water table angle is 90 degrees, when a little thought shows it just cannot be so.
Though the barrels assembly is static and apparently simple, it is not. Just try and draft a pair of barrels and you will discover just how complicated things get, ie convergence, alignment on the horizontal plane, firing pin distance, the ejector leg hole location and the metal thickness between that and the chambers, a whole bunch of parameters jump out at you.
One of the most complicated projects for me was the fitting of monobloc barrels onto an action designed for chopper lumps. The two methods of construction differ at critical points and it is a headache to get it all correct.
