A little thing called the second amendment to the Constitution. You are free to pass another amendment to overcome that if you choose.
The second amendment does not prohibit licensing. You really need to start reading the Constitution as it was written, not the one your unicorn-infested mind keeps coming up with.
And speaking of the actual Second Amendment, those who cherish it should be leading the movement for some sort of tighter restrictions, because we are only a few more school massacres away from a liberal fomented move to remove it from the Constitution. And if the amendments defenders continue to make the argument that the second amendment makes us powerless to put limits on this stuff, that movement will be successful.