I have and 870 with a 23" long rifled barrel that has the cantelever scope mount. I mounted a Nikon Prostaff 2-7x 32mm shotgun scope on it. Cost was about $129.00. I am extremely happy with the whole package. Perfect scope for that firearm, it's clear, crisp, and very accurate.

Check with theopticzone.com, they seem to have the best prices.