Greg, seems that you have put in a considerable amount of effort to reach your goal, so I applaud your efforts. No 2 guns tend to be exactly the same nor do people. A rock solid gun vise might eliminate the shooter for the most part and then maybe you can get a good analysis of the gun itself. More importantly the barrel. You know you are shooting into 1% or less, so tuning isn't the issue. That basically leaves the barrel and or pellet.
Barrels are damn near like snow flakes. Every barrel is somewhat unique to itself. There really is no such thing as 2 identical barrels. Many barrels are good, some not so good and very few are exceptional. I have slugged and measured alot of barrels across many different brands and that really shows the variance in barrels and makes it easier to understand why some guns are more precise than others. At one point in time, I had 4 different brands of .25 barrels that were all machined for the exact same gun. With the same tune, the gun would shoot a different velocity and level of precision with each of the 4 barrels. One could say that is expected since all 4 barrels were different and I agree. However, I also had 3 barrels that were from the same maker. Twist, length, and caliber all the same yet 2 barrels shot to somewhat the same level, but the 3rd was by far better than the other 2.