As Brian Doherty recently noted in Reason, officials in California are using that state's gun registry to "visit" known gun owners who may no longer be allowed to own guns. They're not going to take everyone's guns, just the ones from those people who are no longer allowed to own them. It's not like they're seizing all guns. At least not now. But they know where the guns are, because California has registration.
the government has a list of where the guns are, then they can take
They can take them if they think someone is a felon. Or they can
take them if the person seeks mental health treatment. Or they can
take them if that person lives with a person
who seeks mental health treatment. Or they can take them if the
legislature decides that that type of gun is no longer legal to possess.
The point of background checks is to have a list of people who aren't allowed to purchase a gun. When someone tries to buy a gun from a firearms dealer, that dealer checks the list to make sure the purchaser isn't prohibited from buying a gun.
However, the point of registration is to know where the guns are and who owns them.
To be able to take them.