A few questions first, are you shooting rifled or smooth bore and what is the twist rate in your barrel?
I have a hastings rifle sighted barrel on my 12ga 1187 1:34" twist rate, slower twist rate barrels work best with slower slugs generally 1500fps and below. I have found the best groups with Winchester Super X 3" Sabot 1400fps and Lightfield Hybred EXP 2 3/4" 1450fps. With either of those I can shoot 3 shot groups at 50yds and have the holes touching.
The faster such as the Hornady SST 2000fps generally tend to work better in the faster twist rate(1:24") commonly found in newer slug barrels. A buddy of mine shoots those from his Mossy 930 with the factory slug barrel and loves them.
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