If they try going door to door, it's civil war. They may try a buy back to reduce #s. The only thing that will accomplish is a turn in of seriously broke dick guns or guns from people who we don't care about. They only people we care about are the ones who take care of their weapons and would never sell them back. In other words, that would just weed out the weak hands within the gun owning community.
Do the math...
If every federal agent was dedicated to searching homes...and everyhome was unlocked and there was zero resistance...and they worked 24/7 nonstop....it would take decades. There's 350 million homes in the US.
NOT to mention all the trunks, office ceilings, pontoon boat, grain silos.... There's no way to search. It has to be voluntary and punative.