If there are no mandatory background checks, then it's legal to buy/sell your guns without a paper trail. It is legally possible to have guns that the government doesn't know about, or to dispose of them without the government knowing about that.

That makes a registry of limited value. If they ban a gun you own, and they come looking for yours because it was in the registry, you can say "I don't have it any more", and that will not be a confession to a crime that is punishable by jail time and a lifetime ban on gun ownership.