It will do OK. The parrallax will be wrong most likely set to 100 yards. Most shotgun scopes are 75 and rimfire 50. Most likely you will not notice a difference. Eye relief is also something to look into..
^^^^^ Eye relief is what I would worry about. Just make sure you mount it as far forward as comortable, and you should be fine. I don't hunt with a slug gun anymore, but I liked 4" of eye relief or more.
That should be fine, I have a 3x9x40 simmons on a Ithaca model 87 slug gun. It's been on there for around 15 years and not one problem.Just remember the RECOIL or you will have a half moon over your eye!!