The limiting factor will be accuracy with a smoothbore shotgun and a rifled slug. Most that I have had any dealings with I would not have trusted past 60 yards. Don't go by random stories of one miracle gun on the internet. Go to the range and test your gun. It's the only way to know. The slug itself will maintain killing energy well past the range you will be able to shoot a smoothbore accurately.

A saboted slug through a rifled slug barrel is another matter and is typically much more accurate. With a scope on a rig like this 150 yards is easily doable and 200 yards is not unheard of.