re: What stops the government from coming door to door and making you surrender your guns

In reality, you probably couldn't stop them, at least at first. If there was enough popular support to keep the guns and people were outraged the public could do things like serious road blockages and stop the interstate commerce, but on an individual basis probably not much. If it came down to that I believe the lines in the sand would already have been made and a more brutal response from the government would be expected. Think pre WWII Germany.