If you have a pilot certificate valid for rotorcraft, you can legally fly any helicopter. You don't need an ATP, nor do you need a type rating in a heavy aircraft to fly it. You need the type rating only to be the pilot in command. For anything rated less than 12,500 lb, you can fly any of them as PIC as long as the owner allows it. You can buy your own S76 and fly it yourself if you have the money, with no additional training. It's not smart, but it's legal. It's also not often done, because the insurance company won't cover you. Many things are prohibited by economics rather than regulation. Plus, if you have the money to buy and fly a medium helicopter, you probably have enough sense, and money, to get proper training. But maybe not...