A repeal of the Second Amendment.
Only active duty army and active duty police (and you could argue the latter) should be allowed to have guns. We don't need militias anymore.
And what exactly happens to the hundreds of millions of firearms currently in the USA? And the millions of people who'd rather be shot defending their guns than have them taken away?
I'd propose paying people off in exchange for their guns. Like a gun buyback.
As for the millions of people who would rather be shot...either draft them into the military, where they can use a gun (as long as they're there anyway), or just give them what they want and shoot them if they won't surrender their guns peacefully.