RE: 870 rifled barrel or not...?
If all you plan to use the barrel for is varmits, I suggest getting a .22Magnum Rimfire. You can get the gun for almost the cost of a new barrel and more accurate than the sluggun. You might not retrieve all the animals but your not looking to retrieve them all. You can get a 870 barrel with rifle sights for around 130 dollars. The cantelever models are around 200 dollars.