What they'll do is make it a penalty -- if they know you have guns and you don't turn them in, they won't come to your house and provoke a confrontation. They'll just use the IRS or some other mechanism to sweep your accounts. Turn in your guns, or feed your family.
That's why the leftist claim that "nobody is going to come to your house to collect your guns" is irrelevant. There are other ways to force (or at least very strongly encourage) compliance.
...and dry up ammunition supplies driving up the cost.