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.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra