Nancy Lanza was delusional. Her son was mentally ill. There's no way she should have been allowing him access to any firearm and probably should have been concerned that he had any association with weapons, including the fantasy world where he slaughtered fictional characters in a video game...
Restrict access to so called assault rifles (want to shoot an "assault rifle" go to a licensed firing range and rent one for an hour), limit magazine capacity to no more then 5 rounds, do a much more rigorous background check on anyone buying handguns, and ask the obvious questions; is anyone in your home mentally incapacitated, do you have children living in your house.
The suicide issue isn't persuasive because anyone who really wants to commit suicide will find a way.
If someone is really concerned about violence in their home, get a dog, and if you're not satisfied with just a dog, buy a 410, while the intruder is busy with the dog get your shotgun, but if the dog is fairly large and loves the homeowner the shotgun probably is not necessary.
