Shotguns are very picky on what ammo they like or shoot well. I have had many cases where trying different ammo where the impact points were several inches away from the last ammo. When I test ammo I give no concern to where the bullets impact in regards to point of aim. I only worry about how they group. When I find one I like I reset my zero for that ammo.

And like said above you compounded things by shooting rifled slugs through your gun first. Usually they foul the barrel after 5 shots or so. Before you do any testing with any other ammo, especially high dollar sabots you need to clean that barrel VERY well. Get some good solvent and plan on spending some time scrubbing. I actually chuck my cleaning rod up in a cordless drill when I do it. Just be careful when doing it and don't do it longer than you need to.

Paul