I would not spring for a Slug model or fully rifled barrel on a 12 GA.
Smoothbore is the way to go with a slug gun. 20GA would be a better option.
If you do spring for the rifled barrel slug gun 870 pump action. Go for it in 20 GA Ammo for this gun in 12 or 20 GA rifled barrel is going to be pricey same applies to the gun. shoot for a 1-30ish twist rate barrel faster then that will cost you well into the thousands. Grab some sabot ammo also not cheap 1oz pure lead slugs with a plastic sleeve around it for your GA of gun get it in 2 3/4 if your gun can use 2 3/4 or 3". keep the FPS around 1600 or lower no faster it will shoot bad skips rifling if you go faster in that twist rate. See this it may help you or better explain why I recommend the above Slug gun Ammo & Muzzleloader Ammo.AVI - YouTube
P.S. If you want to shoot the 2200 FPS ammo like accutips you will need a 1-28 or faster twist barrel Very pricey. You may also want to look into muzzleloaders you can hunt the special season for em or use em in shotgun also in most states. I also have videos that give a full rundown on them. Check em out Hope this helps GL HF Happy hunting & be Safe