Apples and oranges. It is happening, right now. Just one for instance that is in the news now: Hawaii has decided to defy federal law and make medical marijuana legal, with a prescription. But now, that same state government has sent letters to people with medical marijuana cards (a violation of patient privacy) saying they have to turn in their guns because they are in violation of federal law using marijuana and owning guns, so they have to turn them in. Now Hawaii Five-Oh has announced they will be confiscating guns from those that don’t comply.
A more egregious example is that the VA has said that people who get VA benefits, but have elected someone else to handle their VA payments are by definition not competent to handle their own affairs and therefore not competent to have guns. They have reported all them the to the BAFTE. Those people cannot pass NICS checks now. BAFTE is required by law to confiscate and they have done some. Fortunately BAFTE is understaffed and grossly incompetent, so it has been limited.
After Katrina, law enforcement conducted unlawful searches and seizures and forcefully interrogated suspected gun owners on the say so of the mayor of New Orleans. Guns were confiscated and never returned. New Orleans was sued over this and lost.
Hitler said that to achieve his aims, there could be no guns in private hands. He started with registration, which enabled confiscation. You either learn the lessons of history or you repeat them. If it can happen, it will happen.