Let's look for common denominators in well-publicized shootings like Trolley Square, the mall in Oregon, the school in Connecticut, or the mega-church in Colorado. An "assault rifle" is not one of the common denominators, but rather they are: 1) a mentally ill person, who 2) has frequently been deeply involved with violent video games, and 3) has gained access to firearms, legally or illegally. So perhaps our discussion should have three aspects to it rather than just one.
First, better identification and professional treatment for the mentally ill. The availability of full-time treatment facilities has plummeted nationally by more than 75% in the past two decades. There is a stigma in our society to mental illness. Medical insurance programs provide scant coverage for it, there is no way to force patients to take their medications, and information concerning it is extremely confidential and rarely reported beyond the immediate provider. Utah, for example, does not share such information with the federal data base that is used for background checks before purchasing firearms. Is mental illness a common denominator that begs to be addressed? Yeah, but it's hard/expensive to do, and raises privacy concerns, so let's move on to something easier that's less effective.
How about video games? They come in two basic modes: race and crash, or shoot and destroy. Have you watched any of the most popular games that junior high and high school kids play? You get more points and more powerful weapons for the more enemies that you destroy. An hour or two a day, every day of the year, and what do you think that does for desensitizing our kids to violence. Watched "Django," "Zero Dark Thirty," or Bruce Willis or Arnie movies? Ah, but we don't want to infringe on profits, limit choices, or deny our kids what their friends are doing. So let's keep moving to something even easier and even less effective.
OK, so let's put more restrictions on legal firearms. Ignore the soaring murder rates in cities that have draconian firearms restrictions, and ignore the dropping crime rates in states that have fairly liberal concealed-weapon laws. Oh, and did you know that no "assault rifle" was used in the Newtown school shooting (he used four handguns and left the AR-15 in his car)? To paraphrase Al Gore, those are "inconvenient truths." Let's instead pick something more visible and largely irrelevant, like assault rifles (not sure what they are, but they sure do look evil!) and large-capacity magazines. That's easy to do and will make us feel good, so let's climb on that bandwagon.
Let's avoid things that can really protect our kids and our society at large and just "do something" symbolic, like maybe a letter to the mayor of New York that says we're against illegal guns, or make everyone except criminals register firearms, or ban something that looks evil. It's really just too hard to do anything that would make a significant difference.