Perhaps 50-100 million firearms currently owned by law-abiding citizens will become contraband with the stroke of a pen. Citizens will either register their firearms, or turn them in to agents of the federal government, or risk becoming criminals themselves. Faced with this choice, millions will indeed register their arms. Perhaps as many will claim they’ve sold their arms, or had them stolen. Suppose that as many as 200-250 million weapons of other types will go unregistered.
There is not going to be any "registration", period. The next step would be confiscation and from there it's just a short stroll to a camp for "re-education".
The 535 members of the House and Senate in both parties that allowed such a law to pass would largely be on their own; the Secret Service is too small to protect all of them and their families, the Capitol Police too unskilled, and competent private security not particularly interested in working against their own best interests at any price. The elites will be steadily whittled down, and if they can not be reached directly, the targets will become their staffers, spouses, children, and grandchildren. Grandstanding media figures loyal to the regime would die in droves, executed as enemies of the Republic.
You can expect congressional staffs to disintegrate with just a few shootings, and expect elected officials themselves to resign well before a quarter of their number are eliminated, leaving us with a boxed-in executive, his cabinet loyalists trapped in the same win, die, or flee the country circumstance, military regime loyalists, and whatever State Governors who desire to risk their necks as well.
Most of the police and military will abandon their jobs rather than follow a unconstitutional law. Going on a raid to confiscate weapons would be a suicide mission. The ones left would be wondering who is going in first, and surly die.
Write your Senators, both of them
Write your Representative, write the ones on either side of you too
Hell, send them a link to the article. Maybe they will get the message, or maybe not.