Although I don't doubt that some people would go down fighting if the gov comes knocking at the door to the guns, it will be a very small minority and survival of such a feat will be zilch.
Most people will hand over their guns willingly.
Many will have some stashed away that where purchased a while back or on the street.
However there are so many guns on the street in the USA that this will never work 100% and the amount of time, effort and personnel would not be workable on a large scale.
If and when such a thing would happen it would start with laws on possession with extreme penalties and you'll have to turn them in at designated locations.
To me what is more worrisome is the smaller laws restricting certain firearms one at a time.
And making it illegal to transfer or sell like passing them on in a will and such.