In 2007 (seems to be the last comprehensive effort to make an estimate), there were 270,000,000 guns in the United States of America possessed by civilians. Except for the relatively small number of fully automatic weapons (i.e., machine guns), these guns are unregistered. We might be able to find out who the guns were sold to originally, but as we go further back in time that becomes harder, and in any event as guns are sold or stolen the task of further tracing ownership becomes obviously impossible. Imagine if we suddenly decided to confiscate television sets when almost everyone who owned one didn't want to give them up. That would be the kind of effort that would be required.
I can't help but get lawyerly and add: Even if the Supreme Court decided the Second Amendment didn't protect an individual right to own a gun, confiscating guns would be a taking under the Fifth Amendment of the United States Constitution. How much do you think those 270 million guns are worth? That's how much the United States government would have to pay out to gun owners whose weapons were confiscated.