Using 00 buckshot or a rifled slug, either a modified or open choke is what you need in a smoothbore barrel. If using a rifled slug barrel, I suggest using sabot slugs. If you are planning on 50 yard or less shots, there is no need to go with a scope. If you want to go up to 150 yard shots, get a rifled slug barrel with a scope and use sabot slugs. I have a cantilever rifled slug barrel for my M-870 with a Bushnell Elite 3200 3-9 x 40 scope that I shoot Winchester Supreme Elite XP3 12 ga. 3 inch 300 grain sabot slugs. This with a 100 yard zero, it hits 1/2 inch high @ 50 yards and 3 1/2 inches low @ 150 yards. These slugs usually will expand to about .90 caliber. Again, if you are planning on 50 yard or less shooting, there is no need to go to all this extra expense.
If you want to go all out, the best price for a cantilever rifled slug barrel is Cabela's (
www.cabelas.com) and probably the best or one of the best priced places for scopes and rings is Natchez Shooters Supply (
www.natchezss.com). I recently speced out mine for a Minnesota deer hunt this coming season that I have been invited on. They can only use shotguns in that part of the state where I am going.