I don't agree with banning semi-automatic rifles or handguns, but I do think they should be more strictly regulated and perhaps even more hoops need to be jumped through in order to obtain them. Many of the mass murders committed were done so with handguns (in fact, handguns used in these sprees were almost double of assault rifles).
The underlying issue with all mass murders is mental instability. A majority (at least 75%) of these mass murderers had prior documented mental illnesses prior to their killing sprees. Do we blame the gun in this situation, or do we blame the lack of proper mental health care? I side with attacking the root of the problem, which is how shitty our care system is for those with mental deficiencies. My mom was recently laid off from a state ran mental institution which released 100 out of 500 patients to other care centers, while the other 400 or so were released out into the public. The reason for shutting the doors to the hospital was the state decided they couldn't fund the hospital any longer, so 400 people with mental deficiencies were just released back into the major population. They became free to obtain handguns, free to drive a car, free to do anything they wanted unchecked. Have they been taking their prescribed medications? Who knows. The state decided to declare them all mentally fit in order to justify shutting the doors. If you asked my mom if those people were stable enough to return to society as normal citizens, her opinion would be a resounding "no". If we as a society can't see the problem in that then we have bigger issues than banning weapons. Mental health has become something we as a society have pushed to the side. We pretend it's not an issue and hide from it. We are embarrassed of our family members who portray signs of mental issues, we laugh at the "crazies" and their odd behavior, but we never try to actually address the problem with the same fervor we address firearms. I know that people have touched on the fact that other countries have just as many mental issues as the states so I'm not trying to rehash that, but I think the combination of the readily available firearms mixed with a lack of care for those with mental issues is a volatile. I think both angles need to be looked at equally.
A Guide to Mass Shootings in America | Mother Jones
I personally don't think we should have an outright ban on all assault type weapons, but I also wouldn't oppose further regulation and treating them the way we currently treat fully automatic weapons: Select registered weapon from Class III FFL; apply with both federal government and local chief law enforcement officer which both must approve; apply for permit; ) The stats in our own states don't lie, some of the highest gun crime occurs in areas where guns are completely forbidden (DC, NY for example). Here is a good article regarding gun ownership in other countries:
Harvard Study: Gun Control Is Counterproductive | The American Civil Rights Union