Interesting question! At 50 yards? I'll have to go with .44 mag. That's based off an accurate shot placed in the kill zone, though. If I'm supposed to consider the likelihood of a "reasonably practiced shooter" making a good shot, then I don't think there's a pistol caliber on the list that would be more effective than a bow at 40-50 yards if the pistol had a 5" barrel and iron sights.