While I feel sorry for this guy - several things about this story bother me:
1) Grier (the homeowner with the AK-47) admitted he never saw a gun (or any other weapon).
2) The first time the guys that were "a threat" ever said anything or made any threatening remarks, according to the article, is after Grier has run into the house and grabbed the AK-47 and then gone back outside to face the guys.
3) Finally, he fires 4 warning shots into the grass and the "20+" "MS-13" guys that were there either take off or give up.
I hope there's more to this story but I believe this guy had little or no training in gun defense and, if he really believed they were MS-13, little or no common sense. Why didn't he just lock the house down and call the police while waiting patiently inside with his AK-47 at the ready? What did he think they would say when he walked outside with his AK bearing down on them - especially since there had been no threat posed up to this point? Lastly - if there really were "20+" MS-13 gang members there he was outnumbered and probably easily outgunned if they had decided to do anything. He's lucky to be alive - unless of course he's Charles Bronson.
I hope we get more info on this down the road. I just did an internet search and can't find anything except the original article.