I have the Lee 12ga 7/8oz slug mould. It is not what I would call a high volume mould, but they shoot pretty decent. My wife was able to get 4" 5 shot group standing. The firearm is an rem 870 with a factory fully rifled barrel.
I also have the Lyman 20ga mold. Once again, not a high volume mould. I am shooting it out of a M1100 LT20 with a smooth bore. The jury is still out on this. I am trying to get a rifled slug barrel for it, but can't seem to find one in my price range.