Hey Killer you might want in on this. LOL
Personaly Shooting targets at 100 yards with scope vs Iron sights is a tad different.Ican hold the pattern better with a Scope. But I have with Iron sights kept every shot in a 4" radius of the center bull with no problems at 100yards. It depends on how well you can shoot with one or the other. I know guys out to 200 yards can almost out shoot me with irons sights vs my scope. Eye sight is a major key. If your iron sights are a little fuzzy when closeing one eye to aim get glasses or us a scope.
I shoot a lot of WWII era guns such as 91/30 Mosin Nagant. Lee Enfield. Swiss K31, Mausers, Etc. No scopes. I take them hunting at times. But prefer a scope.
Have no experience with Shotguns w/ Slugs. But my sidelock is pretty accurate at 100 yards. With sabots vs. Roundballs.