The NRA fight adamantly against a national gun registry because they believe it's an infringement of privacy. They seem to be successful in blocking any attempt at passing such a law. We can have a voluntary registry, but it's tough to get people to go sign up if there's no benefit in them doing so.
But what if we create a national "not it" registry to indicate that you no longer own the gun? That way it's not up to you to register, but up to the previous owner to deregister. A gun manufacture can say that these batch of guns have been sold to this dealer thereby relieving themselves of questioning if the guns were used in a crime. This is voluntary, but if there's no record of a transfer, then they're the first line of questioning if the guns are used in a crime. When the dealer sells a gun, he can also register and say who owns the guns now. He can say "f**k you, I'm not going to register anyone," and that's fine. Except that if a gun is used and he's the last owner then the police can come and question him. The same line of logic applies to personal transfers.
This would essentially help create a national registry, except the effort would be shifted to the people who would benefit from taking action rather than the new gun owner.