It's really going to depend on how well you shoot it with open sights. A scope doesn't make a rifle or barrel more accurate, it just lets you see the target better. I have seen some guys with open sights shoot better than guys with scopes, just depends on how well you shoot.

I would advise sighting in at 50 yards, heck start at 25 yards if you want to. Then once you get it dialed in at 50 yards try some shots at 100 to see how you do. Keep in mind wind drift is pretty good on a standard slug so if it is windy out expect your windage to be off at longer distances. Not so much of a problem with some of the newer high end saboted ammo though.

If most of your shots are going to be 75 yards and in with an occasional shot at 100 I would not worry about using open sights, you will probably like them more. Much better field of view and quicker to get on target. Good fiber optic sights are even better yet.

Depending on the ammo you choose sight in a few inches high at 50 yards to compensate for the drop at 100 yards.

You can use cheaper rifled slugs out of a rifled barrel, but they will make a mess and you will have to do some scrubbing when done. Pick some of the moderately priced sabots and you should be ok. If you want to shoot a full bore slug take a loot at the Remington buck hammers. They kick like heck, but they make a reduced recoil version as well.

My opinions any way.

Paul