If your argument is that people need guns for self-defense, you're horribly misinformed. See the El Paso shooting: 22 people dead, in an open carry state. Or, any forensic pathologist will tell you that owning or carrying a gun just increases the risk that you yourself will be shot with it. The stats back this up.
If your argument is that more guns don't lead to more violence, you're horribly misinformed. Gun violence tracks linearly with gun ownership. Worldwide. In every US state. The more guns that exist, the more murders are committed with them. The stats back this up.
If your argument is that someone with the intent to shoot people will buy a gun by any means necessary, so banning them won't help, you're horribly misinformed. States with tighter gun control laws have fewer gun-related deaths. Controlling the sale of guns directly leads to fewer deaths. The stats back this up.
If your argument is that the second amendment guarantees the limitless right to bear arms, you're horribly misinformed. It was interpreted this way not in 1787, but in 2008, and then only in the context of a single handgun for self-defense as part of a Supreme Court decision. I'd prefer to ban all guns, but sure, keep your handgun.
Buying and selling automatic weapons is indefensible. These are weapons of war, designed to be wielded by trained military servicepeople. We don't need them on our streets. It's not about mental health; it's not about drugs; it's not about videogames. It's not about prayer in schools. It's about limiting access to instruments of death.