Half of the guns aren't in the registry; they're only halfway there. The second half will be harder to get than the first half. If you want the second half--and if you're going to make this thing work, I think that has to happen--be prepared to spend more than the $2 billion you spent the first time, because people won't come forward anymore.
In my office we get calls two or three times a week from people: “I was going through Grandad's stuff. I found this old World War I handgun. What do I do with it?” We tell them to take it to the police department. Then when they ask us, “Will they register it?”, and we tell them, no, they'll destroy it, they're not allowed to register it, we'll hear them say, “Oh. Okay. Thanks.”
Click.