I don't shoot the smoothbore 870 ......But I have the SP870' 3" magnum, 24" fully rifled heavy barrel, canatilevered scope rail w/3x9 Leupold Vari-X III. I shoot Remingtom Copper Solids or Remingto Ultra Bonded and can easily put 5 rounds in a 6" paper plate "standing" @75 yards and can do the same from sitting, prone or a "casual" rest out to about 125 yards. From a bench @ 50yards you will get about 2" and 3 1/2" to 4" Inches @ 100 yards(factory claims under 3" @ 100 yards but that is not realworld IMO).
If you want to shoot "Traditional Lead Slugs" use an open a choke as you can find (Cyl Bore or Imp Cyl) if you shoot Sabots use a rifled choke. Shoot different brands of slugs as different guns seem to do better with certain ammo (this will be much more dramatic than with any rifle you have ever shot). If you don't want to get another barrel I would recommend trying a rifled choke tube and Remington "Buckhammer" slugs. The Buckhammer slug is suppose to be designed primarily for smoothbores w/rifled chokes. I have never shot them but have heard good things from those who have.
I shoot smooth bore....1100 remington, and sawed off sears and roebuck (my old reliable)....I have found the ammo is the key, the most consistant shooting slugs for me are federal....we (friends) shot federal, remington, brennican(sp),and winchester slugs thru 4 different guns and the federal were the best shooting in all the guns(most consistant) in a box of 5 the remington would have one that would shoot high and right 8to 10in....the brennican(sp) all shot high and right about 10in., the winchester were all over the place.This was the same in all 4 guns!!!with 2 different shooters
I have killed deer useing remington slugs, but after the little test I now only shoot the federal,(and have not missed yet)we can only gun hunt in Ohio with shotgun and slugs.....except you can use handguns 357 or larger,plus muzzle loader....go figure....but that slug will knock them down. most slug barrel's(smooth bore)are improved cly. I use cly in my 1100. Good luck I hope you get that big one....:angel
and if you use a scope check the tightness of your rings every year....mine wear loose this year....:banghead... just sitting in the safe