Thanks again Si and Arms Dealer, excellent work and a lot of data to study there!.
On first inspection, I would have said that string 6 was mega consistent over the chrono downrange but your comment was that the group wasn't good and you wouldn't have chosen that die as a keeper.
The weight of evidence is building up to suggest that this theory isn't a good one.
It does lead me to some other thoughts - if the theory doesn't hold up, then neither do the calculations used in ballistic calculators. Which is a bit of a shocker I guess