Your range will depend a lot on what specific slug you are shooting and wether or not the bbl is rifled. Try different brands of slugs to see which groups the best in your gun. I have had very good groups with the cantilevered bbl on the 870. The best slug load in my particular gun was the Remington Copper Sabot slug.
i have an 870 with the scope mounted to the side of the receiver. http://www.combathunting.com/SCOPE_M...ope_Mount.html with a smooth bore and a rifled choke tube (originally used im. cyl. barrel with almost exact same results). i shoot remington rifled slugs. they group 2.5" at 75 yrs. killed a fair amouont of deer with set up.