I really don't appreciate you guys saying things like "it couldn't happen" and "it's just a movie". Tom Selleck is one cool mofo, he can do anything. His list of skills extends well beyond shooting buckets at 600+ yards. Next thing you know you guys will be saying Chuck Norris is not the baddest SOB on the planet.
All kidding aside, it was a great movie. And while the conditions were less than optimal for the shot(unknown range, offhand, snapshot), they at least somewhat set the premise up that he was the best. So its not like it was some rancher with a 30-30, this guy had a specialized rifle and basically shot for a living. Not that it could be done, but if it could be, they at least made it believable that this would be the guy capable of it.