SailorCurt's hunch about the crazy son 'pretending' to rob his mom and getting shot for his troubles looks like it was spot on.
Son has lengthy drug related problems with the local police, and they believe that he was really trying to rob her. Well, that turned out to be a bad idea.
Mom may have some problems of her own since it was her weapon that had the obliterated serial number. Who knows.
Thanks to SailorCurt for the scoop!