I don't own any shotguns basically, so the simple question is: If I was to grab a basic Remington 870, can I add the slug gun barrel, or is the rifled slug gun specific to only a certain model? Aren't the barrels on these easy to swap in or out depending on specific hunting of the moment?
You can easily swap the barrels on a Remington 870. A fully rifled barrel will have either open sights or a rail to mount a scope. The buttstock will be fine for open sights but after mounting a scope on a barrel with the cantileavered rail the buttstock will be to low for a good cheek weld on the stock. Many slugs do not require a fully rifled barrel to be accurate. If the shotgun has replaceable choke tubes you have the option of inserting a rifled choke tube. Even without a fully rifled barrel or even a rifled choke tube some slugs are still pretty accurate from a non rifled barrel.