A fixed range? Absurd. Ludicrous. Laughable. Dangerous.

Distance as a legal requirement has nothing to do with it. What matters is that you're in immediate jeopardy, and really, that you have no alternatives to lethal force--in other words, that you can't safely escape or avoid the danger.

Note the "safely" part. If a guy is threatening you with a knife, and you reasonably believe that he really means it, I don't see the wisdom in turning around and running if he's 50 feet away. By all means, back away, but don't put yourself in further risk trying to avoid him. The folks that write "duty to retreat" laws seem to have a hard time understanding this. It's almost a contradiction--if lethal force is justified, then retreat is either not possible or unsafe. The fight has started.

Anyways, semiautomatic rifles have a wide array of advantages. They've got lower recoil and better capacity than a shotgun, and are generally easier to be accurate with than a handgun. But really, the shotgun or AR you take out twice a year doesn't hold a candle to the pistol you shoot everyday.

The fact that 5.56mm is considered effective out to 600 meters is incidental.