Don't limit yourself to rifles, get a slug gun and give it a try! Two types of slugs out there, foster (rifled slug,) and sabot. You use foster/rifled slugs in smoothbore barrels (normally modified choke or improved cyclinder work best IMO,) and you use sabots in rifled barrels. Sabots do shoot better, but are much more expensive. You can use any off the shelf bird gun as a close range (about 60yds and under,) slug gun. You could also get two barrels one with rifled barrel and sights and the other a bird barrel.
Most guns now have screw in chokes and you can use a rifled choke with some slugs (see manufacture charts, Remington has a good one,) and put on some type of red dot or scope in a regular mount or even rifle type turkey sights onto the vent rib. I haven't tried them, but I hear that Federal Truball and Remington Rackmaster slugs used in smoothbore or a smoothbore with a rifled sight are just as good as sabots in a rifled barrel.
As for water buffalo, I have no idea. Maybe a rifled barrel with partition gold winchester. Way out of my league on that question. Slugs are usually soft lead and don't hold up like a true penetrator load in a 375 or 458 rifle. I met a guy who was torn up by an african buffalo. He only lived because there was a couple of doctors on that guided hunt. No way I'd use a single shot shotgun, maybe a 600 nitro!