It is not at all uncommon for a clean barrel to shoot to a slightly different POI than a fouled one.

Have you tried shooting groups by swabbing between each shot with 50/50 alcohol and windex, then a dry patch? Basically you are shooting a clean barrel each time. If the gun shoots good groups this way, sight in for that POI.

If it won't group from a clean barrel, you could try other sabot/bullet combinations. If you still have trouble, you could hunt with a fouled barrel, but that is the last resort IMO.