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.