RE: sighting in a scope
A few things:
1) Contact your manufacturer and see what they recommend shooting through your gun/barrel. The manufacturer tries ALL the major ammunitions, and usually have a good idea of where to start for Brands/loads.
2) Sight in at 25 yards. You want to be dead on horizontally, and ~2-3 inches high. Of course, this will vary on your gun, load, and scope---but in general, you will see about 1.5 inch drop/25 yard interval. That puts your gun dead-nuts at 75 yards, and ~2 inches low at 100. Once you are hitting good groups at 25 yards, check your groups at 50 yards and further. However, many "experts" recommend to not adjust your scope when the slug is travelling less than 1500fps with a 12 gauge, and less than 1000fps with a 20 gauge. With most slugs, therefore, you shouldn't adjust your scope past 50 yards.
NOTE: Notice I said "adjust your scope"---by all means definitely target shoot past that distance and determine how your gun groups, if you plan on taking 50+ yard shots.
3) Let your barrel cool between each shot, or shoot two shots in quick succession. Slug barrels can do finicky things with slugs when they are "hot".
4) As previously suggested, you may have to clean your barrel after every 2-3 shots. I think between every shot is a little extreme, but depending on the load you may get some excess fouling which affects the grooves.
5) As also previously mentioned, a steady rest is best when doing the initial sighting-in.
6) Perform a google search...there are hundreds of well-written articles on the internet about properly (and improperly) sighting in a scope, on all types of firearms.
Good luck...
S&R