Both barrel types preferably
I am looking to buy a shtgun and am debating if I should go with the regular barrel or belly up and buy a combo of rifled slug and regular barrel.

Thanks in advance

I'd say 50-75 yd for a smooth bore barrel, (although some smooth bore shotguns can group at 100 yd with the right slugs). Maybe 150 - 200 yd. for a rifled barrel with saboted slugs.

Best thing to do is to pattern your gun at different yardage and then determine what range your gun will allow you to make an ethical shot on an annimal. For me, I don't shoot at a deer beyond 30 yard w/ a shotgun, 25 yds w/ a bow. This keeps my recovery rate high.

As far as what to buy, it will depend on where you plan to shoot/hunt. My first shotgun was a combo with a slug barrel and a smooth barrel. Because I can use rifles where I hunt, I only use the smooth barrel. For distance shot I use the ML or rifle.

From my experience with shotguns, it comes down to accuracy. A smoothbore barrel won't be too accurate out past 50 or 75 yards I don't think. If you get a rifled barrel and try out some of the newer sabots, I think you will find you can hit a paper plate (about the size of a whitetail deer's vitals) at 150 yards without too much difficulty. The slug is starting to drop pretty fast out there at that range, so practicing at 100, 125, and 150 yards would be important.

Regular smoothbore barrels are not that good at stabilizing slugs, especially sabots. A sabot round needs the spin from the rifling to allow the sabot to separate from the actual bullet. Sabot rounds are what you really want to use for deer, they are way more advanced than the older slugs.

I shoot a Encore rifled Slug barrel, I am comfortable out to 120 yards. I shoot SST by Hornady. Smooth Bore 50 - 75 tops. I can push my encore father but choose not to.

I shoot a Browning with a rifled choke. I shoot lightfield sabot slugs. this combo is accurate to 125 yds. Past that you have to account for drop.

For smoothbore, 50 - 75 yards for most shotguns. My friend can shoot his accurately out to 100 yards, but from my experience, that is the exception rather than the rule.

With rifled shotguns, the only experience I've had is with the H&R single shot full rifled. That was extremely accurate with Lightfield slugs - like 2" groups at 100 yards. It is my cousin's deer gun, and he has taken deer out to 120 yards with it. I'm sure it would be good to 150+ if you practice to account for the bullet drop.

Wow, that's out there.

What brand and model is this shotgun?

And the right slugs? What brand are they? Any particular bullet configuration?

iSnipe

With my 870 28" bbl (smoothbore), I couldn't hit the broad side of a barn at 25 yards with rifled slugs, using the standard Rem chokes. Okay, I could hit a target at 25 yards but about 8 inches high and 5 inches to the left. TERRIBLE. I recently got the Wad Wizard choke and now I can sling rifled slugs 100 yards within 3 inches of bullseye. Go figure!

I also I have a Marlin 512 bolt action slug gun. It has a 3" chamber but I've only fired 2 3/4" sabots out of it. It's sighted in for 100 yards and can do 1.5" groups at this range with the Remington Premier Accutip sabots. They are the ONLY slugs I was able to get to group accurately out of that gun. With that said, I haven't tried everything out there. I am able to group about 4" at 200 yards with this gun. I aim about 3-4" high to hit bullseye.

get yourself the rifled/cantilever barrel a place a good scope on it, try different sabot slugs to see which works best