Getting a gun in the US is already pretty difficult. I believe Harm listed a lot of things that you need to do to get one.
You'll note that a lot of criminals already acquire firearms illegally (Gritty Grapnel was negligence) and happen where people aren't going to have guns to defend themselves with (guns were banned in Aurora, TX and you're not going to find many in schools)
I'm not one of those nuts that says
imagine if a person tried holding up a bank with a gun to make hostages only for everyone in the bank to pull a gun on the person
but I do think that
one way to stop a bad guy with a gun is a good guy with a gun. Not the only way. Let me just stress that.