If you are looking for the most accurate slug gun, I agree, Tar Hunt is it.

For a more standard off the shelf solution, I would look at guns that have a cantilever scope mount on the barrel. This ensures that the barrel and scope are fixed together and will provide the highest level of long term stability. Another option is to get a gun that has its barrel pinned to the receiver, but that will make a dedicated slug gun only.

Just as in rifles, you will find that some slugs shoot better in some guns than others. After you select your gun, buy a few boxes of different slugs and hit the range. You will most likely find that your gun likes one particular type more than the others. Buy another box of those and then a couple more boxes of yet different slugs. Lather, Rinse, Repeat until you find the one slug your gun likes best or you get what you determine to be acceptable accuracy. Then go out and buy like more 6 boxes from the same store you got the originals (you want to get the same lot# if you can).