Eightbore in theory you are right. I've seen doubles that had patterns not together at any point. I had a 20 bore which sent the left bore pattern down six inches and left a foot at 21 yards when you compared it to the right barrel or the point you were aiming at. The right barrel was dead on at 21 yards. The left was not. Those patterns did not converge or stay together. They diverged. At 35 yards the left barrel was useless. Don't know if it was poor regulation or chokes being poorly cut. I sent it down the road.
We assume barrels are regulated but have no way to know this without patterning them. Gun makers use a jig to align barrels for joining and statistically it must work almost all the time. I've seen screw in choke tubes which were certainally cut off bore so much that eccentric choke tubes were required to correct where the pattern was. When in doubt go to the pattern plate.
