First of all, make sure your barrel is tight by seeing if you can physically flex it. When I first got a 500 w/rifled barrel, I went to the range w/Winchester BRI sabots. The group was more like a pattern because of barrel twist from shot torque. Tightening by hand wasn't good enough. I went home and covered the barrel screw w/a rag and tightened it w/vice grips leaving a small air gap between the barrel and magazine. If you do this, do not tighten the barrel completely, leaving no gap. Mossberg's manual says that's a no-no.
My groups at 100 yds. tightened immediately. If this isn't your problem, Winchester and Federal load the BRI style slug and that is the most accurate slug I've found so far. Each gun varies, but that slug is definitely worth trying. Reviews seem to back that up.
Last edited by Game Stalker; 11-28-2015 at 04:16 AM.