Yep your pretty much lobbing knuckle balls out there.....I remember those days well!You must understand that without rifling to stabilize the projectile it makes it difficult to be consistent. Most Foster style slugs are made with groves cut in them to act as sort of a rifling effect in that the groves are supposed to impart a spin on the slug using wind resistance, thus stabilizing it. While better thanthe oldsolid lead slug it's still primitive at best. Your best option is trying to find a rifled barrel for the gun however this is not cheap.

If money is a factor then I would suggest buying a rifled choke tube made for this very problem. While it won't be the same as a completerifled barrel, it will input some spin on the slug.Then you need to find slugs that shoot well out of that particular gun/set up. What worked well out of another shotgun may not work well out of the Benelli.

As mentioned, in conjunction with this, improving your sights will also help your groups. Stay away from the sabot slugs as they are intended for full rifled barrels and they will give you even worse performance than the regular foster type slugs if shot out of a smooth bore.

Good luck....