After putting 30 grains of powder in the barrel and a cloth wad I fowl the barrel. I then load for target. After that first shot, I take a patch which I wet with denatured alcohol and swab down and back, turn it over and repeat. Then I run a dry patch down and up. Then load for the second shot. If you do this, all your shots should be about the same downrange.

The trouble cleaning after several shots I found is, the fowling is building up this entire time. Any change in fowling amount does have the possible effect of changing your group. In fact you report the sabot is hard to load. It will also be hard to load after cleaning unless you let the rifle sit a few minutes. You want the barrel to have approximately the same fowling it would have for each and every shot.

Try your test again and this time swab between each and every shot. Your barrel needs time to cool also. It sounds like you have a good shooting rifle there...