I don't think anyone could write this any better than cayenne8, so I won't try. But I will add a few of my own points that I think support it.
* Not only has a mass murder happened in CA which has amongst the strictest of gun laws within the US, gun massacres have happened in France, which has restrictions far greater.
* These massacres have happened in places that by and large prohibit the carry of guns.
* I read roughly 1-3 stories per week of people using guns to prevent bad things from happening, many times without even the pulling of a trigger.
* My parents' generation as students commonly carried a firearm to school without incident so they could use it recreationally afterwards while today, many children's first exposure to the concept of firearms happens via repeated role playing in violent video games and other entertainment.
* While the USA ranks #1 in _gun_ murders, the USA ranks #111 in murder. Excluding the most restrictive US locales w.r.t. gun ownership would make that number significantly higher.
* By a per-capita comparison of 2009-13, Norway, Finland, Slovakia, Israel, and Switzerland exceed the US in mass murder by guns and have much more restrictive laws.
* Gun control laws have enabled harm brought to innocent people, e.g. Carol Browne, Amanda Collins, and Shaheen Allen.
* An evil person wishing to inflict mass casualties can do so without a gun: e.g. Oklahoma City, Waco, 9/11, Boston Marathon.
* The same politicians who call for new gun restrictions vehemently oppose expanding effots to control who and what crosses US the border. (Narco terrorists have their own submarine fleet for smuggling to the US. Think about that for a few minutes.)
* Prosecution of gun law violations has dropped by 25% in the current US presidential administration.
* In comming years 3D printing will become more accessible and thereby make the fabrication of gun parts possible without the thought of legal restrictions. (Related note: the San Bernadino terrorists modified guns that complied with the laws at the time of their purchase to function in ways the laws prohibit.)
I have no illusions about the touchiness of this subject. You have the right to your opinion, and I have the right to mine. Mine is that these facts signal that more gun restrictions will not decrease violence, but rather pose risk to increase it due to responsible people having reduced access to firearms for protection. I don't own a gun because of paranoia or a desire to fulfill any fantasy of heroism. I simply would rather have it and not need rather than need it and not have it. I didn't join the NRA because another member encouraged me or because a purchase bundled a membership. I joined after listing to my local representatives talk about gun control.