They don’t say how because it would be too much at once. What they want is Australian style total confiscation. You can’t reason with antiguners because their position is not based on reason or logic. You can’t bargain with them because they have nothing to offer, and if a bargain is foolishly struck with them they immediately try and get rid of the “loopholes” that were part of the deal. They have no sympathy for another point of view and the never stop, ever. The only three endgames are: total confiscation from everyone who is not a government agent (their desired outcome and welcome to a police state that makes the old Soviet Union look like a bastion of freedom) or they kill enough of us for “resisting arrest” so that they win or we kill enough of them and their jackbooted thugs so as to postpone the problem for a generation.
Why do you think so many police departments snapped up MRAPS? They are instruments to control an armed civilian population. They see a conflict coming where a certain fraction of the population is not going to meekly hand in their guns and they want the advantage because the police are sure not going to defend the bill of rights. (Think self forging projectiles if you think that MRAPS are too tough for citizens)
As for Australia, those guns that were turned in were junk. You had better believe that there as some good ones held back.