My 870 shoots Foster slugs well with the mod. choke. If you want to shoot sabot type slugs Remington rifled choke tube for Remington threads. I have both the extended and flush Rem rifled tube for a 12 ga. and the flush tube seems more accurate. I have another brand tube for the 20 ga that is suppose to work with Foster slugs or sabot slugs and it does seem to work well with both.
Most foster slugs are made small enough to pass thru a full choke for safety. Also inside mesurements of bbls vary so a mod choke in one bbl may be a light mod switch it to the next bbl it maybe full depending on bbl. mesurements.