Long range shots. There's good shooting and then there are the lucky shots. If you ask someone about his best shot and he replies, odds are that he's relating his luckiest shot. If you ask a really good shooter about his best shot, he's likely to just shrug his shoulders becasue the lucky shot is of no interest to him.
Putting the face of reality on things, let's run some ballistics. If you start with a period correct gun, it might be something like a .54 caliber with those little, low fixed sights like on the originals, and be sighted in for 50 yards or so. The heroic 400 yard shot would require a hold about 35-40 over the target. At that point, how would one even see the target, let alone hit it, and say it was good shooting? Any takers?
The only conclusion I can draw is that at best the tall tales were very lucky shots, and at the worst, fabrications.