"Back in the day"; when almost everybody's slug gun was a smoothbore, the last smoothbore I used was a 1987 vintage 12 ga. 870 Remington "Special Purpose Deer" model, which had a smoothbore slug barrel with rifle sights. First thing I did was scope it and I used to shoot it at 100 yd. targets to understand how much drop there was at that range. Once I got used to how high I should aim at 100 yds. ( I used to zero it at 50), I could put several 2 & 3/4" Winchester Foster style slugs on a paper plate (about as good as it gets for smoothbores) . After 6 or 7 rounds the leading would start to degrade the accuracy. Tried 3" magnum slugs for a while but the 2 & 3/4" stuff was more accurate and I eventually noticed that deer couldn't tell the difference, so I stuck with the 2 & 3/4" slugs. So find which slugs your gun prefers, practice with them, and don't let the barrel get too leaded up, ( some guns are more sensitive of that than others) . My longest hit on a deer with a smoothbore was in the 75-90 yard range in 1989 and he took a couple more from closer range to finish things. I'm still using the 870 when I'm not in a rifle zone but it now has a fully rifled barrel using sabot slugs for about 15 years now and I zero it at 100 yards. Now at 100 yards I check zero with 3 rounds on a paper plate and all 3 holes are touching or very close. Sabot slugs are not recommended for smoothbores so if you don't have a decision on which older style slug yet, you'll have to see what it likes and then take it from there. Prior to 1987 back to the early 1970's my deer gun was a 12 ga. High Standard which used to love those old Remington Sluggers, which I think are still around. Smoothbores can be a challenge but with the right slug and some marksmanship the challenge can be met. Best of luck.