Two recent incidents should be interjected here:
1. The fellow in Springfield who ran after and chased down a purse snatcher shooting his concealed weapon, apparently in traffic. I say he was in the wrong for doing that but commend his intentions. He will certainly face charges for firing the weapon when and where he did. One fact was that he had no reason to fear that his life was in danger... He was chasing them.
2. Recently near my hometown in Arkansas a fellow walked out to his truck to go to work. Two men attacked him and began beating him (by the way, this fellow turned out to be a martial arts instructor in his 50s.) His son was in the house and heard his dad calling for help. He approached the kitchen window with a 9mm. One of the guys saw him and started toward the door. I'm not sure if this guy was armed or not. The son fired one round through the window, killing the perp. The other one fled, leaving dad roughed up a little, but he was able to interview with police and the news media...
As for a shotgun, there are many models out there on the marked SPECIFICALLY designed for home defense. I don't see how they could sell them if they were illegal.
The key to "self defense" in any case is the idea of immenent danger and fear for your life.
And if someone is coming toward my door with a gun blasting, I ain't waitin' for him to come in...