Stan'
Note that I said it was "Not Totally Reliable". It is not an absolute guarantee that if the barrel is straight it will hit the right place. I believe your illustration of changing the grip bears that out. You did not change the straightness of the barrels as you changed the grip, only the manner in which it recoiled. Likewise, the barrels can be perfectly straight but if the convergence is off a bit the barrels will not shoot to the same point of impact. Also, we have no guarantee the bores & the OD are absolutely concentric, it's where the bores point that is the important point.
It was just a characteristic to investigate. Investigation of mechanical devices is a series of probes based on past experiences. My experience with actually regulating barrels and then checking other guns which the POI issues has revealed some barrrels that were bowed on the outside. I've found wavy ODs on barrels but bores being crooked are constrained by the limits of wall thickness. Belled muzzles for screw chokes are readily apparent. The point of checking the straightness of the barrel is to take you to the next thing to check...convergence. If the barrels are bowed, measuring convergence along the barrels can/will be misleading. High production sxs barrels are likely assembled in fixturing. If they are, it's less likely the convergence is off. If they are wired together for soldering in the traditional way, by eyeball and measuring, bowed barrels are more possible. One barrel shooting on POA and the other shooting off POA is indicative of an individual barrel issue (i.e. crooked choke, bow, etc), not a purely convergence issue
