someone suggested earlier that there would be a civil war if the govt tries to take peoples guns away. Why do people think they can fight back against the govt? I've never heard of anyone taking on the govt and winning...
Many people will not give up their guns. That will lead to arrests and shootings. Then I think you will have families and friends start banding together in attempt to prevent their guns from being confiscated. Then it gets to the point where the local police force refuses to confiscate the arms because it is to risky and they are fighting their own people.
Then the military/national guard would have to get involved. If they had them confiscate the guns in their own areas and potential shoot people they know then you would see mass refusals and defections. If they were moved around say a National Guard unit from Maryland working Utah then the defections and refusals would lessen but you would still have military personnel, many of who are private gun owners anyways, refusing to carry out the orders.
So in short it is not like it would be private citizens v. an intact government. It would be those for taking the guns v. those agaisnt it.