I just shot my slug gun at the Aurora Sportsman's Club. I shot sabots and sluggers through my rifled barrel at 50 yds. I got 1" groups with sabots and 2" groups with sluggers. This was a rem 870 express, cantilever scope mount barrel, and bushnell holosight.
That doesn't sound too bad, and as long as you are limiting yourself to 50 yd shots, you'd be OK, but that may not be acceptable at 100 yds. If your groups are twice as big as they are at 50 yds, you will probably be as big or bigger at 100 yds. So if the gun groups sabots 3-4" at 100 yds, you may see 6-8" or worse groups with the non-sabots, which can mean misses you otherwise might not have had. Of course, it's all conjecture without actually shooting the gun at 100 yds, but the point is.. Why not have the most accurate gun you can possibly have when going afield, as long as it's within your financial means? If you already have a fully rifled barrel, is the cost between those slugs really worth the chance at missing, or worse yet, wounding a deer?
To save money, you can theoretically sight in a gun using a single shot, but it takes a steady rest (like the Caldwell Lead Sled or similar) and a friend to help you out. Here's how it goes.
First, remove the scope cap covers and then get your gun firmly in the rest, adjust until it's centered on the bullseye of the target and shoot one shot. Now without moving the gun or yourself from the sight picuture, have a friend turn the adjustments on the scope so that the crosshairs move until they are centered on the bullet hole that you just made.
For example, lets say your shot is high and to the right, which means that on the scope, you will have to move the dials in the direction indicating LOW and LEFT. With the crosshairs still centered on the bullseye, have him move the elevation dial until you see the crosshair move up and pass horizontally through the bullet hole you just made. Now, again keeping the veritcal cross hair on the bullseye, have him adjust the scope until you see the cross hair move to the right and line up vertically with the bullet hole. At this point, without having moved the gun at all, the scope crosshairs should be directly centered on the bullethole, not the bullseye.
Now reload and shoot again. Theortically, it should be directly in the bullseye, but the reality is that you will probably very close, but maybe off a bit. Unfortunately guns do not shoot in EXACTLY the same spot every time, so there will be some variance, depending on the overall accuracy of the gun. What I would do at this point is shoot twice more to develope a "group", then adjust from there.
I actually will shoot a 3 shot group, make a mental note of the center of that group, and then have a partner adjust the scope until it's in the center of this new group. At that point, you probably are not going to get any better than that, as all guns have a little variance (also known as group size). I would then shoot one last time at the bullseye to double check your last adjustment and you should be pretty darned close to the bull. Total shells shot, 5. Not that bad!
If you are shooting good sabots and have a rifled slug barrel, you should be in the neighborhood of about 2" at 100 yds.