I had a Mossy 835 Ulti-Mag with rifled slug barrel and the only shells I could get to group out of it were 2 3/4 Federal Premium sabot's. I also hear good things about Lightfield Hybrid EXP's their pricey though.
If your not getting enough back pressure with your left hand on the forearm of the gun when shooting a slug gun at 100 - 150 yds it will open up your groups also.
