Australia has a gun registry that is essentially ignored by the majority of its population. I read once that they believe their registration percentage when they implemented it was around 1-2%.
There are certainly unregistered guns, but only guns rights groups with a barrow to push say unregistered guns make up an order of magnitude less than what you are saying - like, 25 times less.
USA may implement it differently or in a less secure way, but it can be implemented effectively.