Going door to door in Chicago confiscating guns HAS been done IN CHICAGO.
Years ago, maybe 10years? the Chicago PD went with the BATF when they did a firearms dealer inspection. They took down the info of people with a Chicago address buying a handgun.
Chicago at that time had a closed registration so you could not own a handgun in Chicago that had not been registered many years in the past.
They went down the list knocking on doors saying "give it up or go to jail". What I heard is most people rolled over when confronted. Because a few did not, the practice came to light during court proceeding.