Stay away from the 3" shot shells. The 2 3/4" is plenty for anything you'll likely to hunt. The main advantage in the 3" or 3 1/2" shot shell is not so much the muzzle velocity of the shot load but rather the total number of shot in the pattern. And that does help for sure on bigger stuff like geese and turkey. As far as using as a "slug" gun. I'd spend my $$$ on a rifled barrel and go that route. With all of these quality sabot type rounds out there now, you can kill deer sized game cleanly out to 125-150 yards.