I agree with Lawdog. I love my semi-auto. A beretta 390 with a hastings paradox barrel. I'm not sure how much you want to spend. I'm a big fan of buying a gun and then purchasing the barrel for it.
I don't really think you can go wrong with any of the guns with a rifled barrel. However, if you are going to scope it, get a barrel with a cantilever mount and my personal preference then would definitely be a semi-auto. Shoot numerous brands of slugs through it until you find which one shoots best and then your are done. Some times for the price of ordering a "slug gun", you could buy the gun with a vent rib, and then purchase a slug barrel for it later for just a little more than the cost of the "slug gun". Check your prices and don't be afraid to look at Hastings slug barrels. I know that to buy a barrel made for my beretta from beretta, it would cost as much as a new gun. Do some research before you buy. Good luck, let us know what you get.