Guns are generally destroyed when collected.
But, the idea of a blanket gun ban law being passed tomorrow (or even ten years from now) followed by a door-to-door confiscation is almost as likely as a zombie apocalypse scenario. If guns ever were banned completely, you'll see these measures implemented slowly... think of the lobster in the slowly heated pot idea.
It might go something like this:
1) Bans on new ownership of certain weapons (ex: assault weapons bans, handguns, etc).
2) Transfer restrictions for assault weapons, handguns, etc.
3) Mandatory registration for all guns.
4) More complicated purchase restrictions (physicals, psychological tests, waiting periods, etc.)
From these four points the noose can slowly be tightened on the gun rights we all enjoy. Some people will simply refuse to register their guns, or turn them in. For failing to do so there will surely be stiff criminal penalties attached to having such guns (like an unregistered machine gun today).
Guns won't be taken overnight by the government agents in black helicopters that so many gun owners dream of. Instead, if guns are ever taken from our hands, they'll be taken slowly, by an ever increasing slew of rules, regulations, restrictions, bans, red tape, and bureaucracy... that's how they'll be taken. Your liberty may walk out the door with those guns, but that won't be apparent to most people until quite some time later.
The thing is, all of the points I mentioned above are among the ideas that the left has floated as "common sense gun laws, which will still preserve the rights of hunters and sportsmen".