What is the barrel configuration? If it has the Invector Plus system, it is back bored and it will never shoot slugs very well. If it has the Invector system,try it with an imporove clyinder,skeet or clyinder choke tube and rifled slugs. If it is a fixed choke with a clyinder,skeet,improved clyinder or ,maybe even modified constriction,try rifled slugs. For best results get a Hastings or Browning full rifled barrel and shoot sabots.
I have never been a fan of rifled choke tubes but I have only a little experience with those things.