The standard S&W revolver comes with a round butt today. For most of the 20th century, they came with square butts, but many people wanted round butts, so they started making them all that way. Reason is, you can put grips on a round but to make it square, but you can't go the other way without major surgery on the grip frame. So, if you buy a new/newish one, it'll have the round butt.
The no-dash 586 I'm currently drooling over (and waiting impatiently for my tax refund to show up) has a square butt, due to its manufacturing date.