Chap - Not sure what you mean by handloading each shot. Don't we all.
During this entire shooting session, except for the very first shot from a squeaky clean barrel, each successive shot was loaded exactly the same. Powder was premeasured at home, swabbing between each shot was the same, bullets were seated to the same depth and shooting style was consistant.
The only difference was the sabots and the amount of pressure required to seat them. The T/C sabots were inconsistant while the Harvesters, without the benefit of a pressure scale, felt the same.
This difference in required seating pressure is what I am sure led to the huge difference in accuracy. Now had the pressures been consistant with the T/C sabots, either tight or loose, I believe that group would have also been smaller.