THEY CAN'T CONFISCATE IF THEY NEVER REGISTER IN THE FIRST PLACE.
There are (At least) 270 million unregistered guns in America. No matter what steps are taken that will require LAW ABIDING people to register them, there will be a margin of NON law abiding people who will not comply. This is the reality which makes the possibility remote enough that practical-minded people won't try it. As soon as they convince us that we must comply 'for the common good', it's all over.