The problem with liberals / gun grabbers is that think they can just pass a law, set up a system, create a database and magically the problem will be solved. To them government is omniscient and omnipotent and can somehow keep criminals or the insane from ever getting ahold of guns with just the power of law, a piece of paper from a court, and a database.
All they would have to do is look at illegal drugs to see just how powerless the government really is. Drugs have been illegal to possess, manufacture or sell for decades. Yet I'm guessing that most of us could easily obtain them regardless of that fact.
So the crazies and the criminals are put in a "no-gun" database. Then the police attempt to "seize" their guns. Since crazies and criminals aren't exactly known for obeying the law, there is good chance they will either lie to authorities and say they don't own any guns or even hide their guns off premises so they aren't discovered if the police conduct a search. And even if the police succeed, the criminals can easily obtain replacements from dozens of sources (black market, straw purchase, theft, etc)
They would have had better luck with SCMODS:
State, County, Municipal Offender Database System