It is my belief your problem lies in the cleaning ritual you've employed, I suspect the fliers are a result of a dirt bore and plastic fouling from the sabots. I have tried just about everything under the sun when at the range and my most constant groups are when I swab between shots. Not only does this maintain a constant bore path or less pressure, but will also a cooling period for the barrel and shooter. I must say near hunting season I will always shoot one and load another directly behind to see how it changes my POI, for me this simulates real field conditions for a possible follow up shot and I want to know how it reacts. I have found very little change in POI with this follow up shot and knock on wood I have never had to employ it while hunting either! Seating pressure may be the result of a flier as well, to help maintain the same seat I suggest making a mark on your ram rod with liquid paper or white marker, this will allow you at a glance to see you are seated evenly each time. To the crushed pellet idea, I have done this by mistake on occasion and can't recall it affecting my POI. While it does stands to reason you could possibly change the composition or seating depth, I have not witnessed any changes personally.

BTW I shoot 100 gr pellets or loose pyrodex with 240 XTP and it perfoms very well. I plan to try 777 loose this year and see what happens. My advice don't bother with the 777 pellets, I tried them and found no benefits other than a bit less elbow grease when cleaning. Like mentioned they are diluted to prevent injury or excess, something I was not aware of until after I purchased a box[:@]!