I would guess the barrel is heating up or you are changing the loading pressure on the sabot with out knowing it. Next time out, try the Shockwaves and shoot, then swab the bore good, and let the rifle rest if it is hot out. I have a shelf under my bench so the rifle can rest in the shade. It will make a difference in the group.

Also try to make loading pressure the same. Some times, as the barrel heats up, the sabots slide easier, so you tend to send them down faster and tighter. Just something to keep in mind.

Actually if the first three are grouping that good, I think you got a real shooter. As for the flyer, even them are not all that bad... Nice shooting.