First off, I doubt most cops and/or military would take part in any consfication schemes.... and unless they could cover every place in teh US at the same time, word would get out ahead of time and people would band together and be ready. It would be a suicide mission for anyone trying to take the guns.
This leaves the only option to be the UN... I don't see that happening..and I sure don't see it going over well.
Gun owners, while unorganized, present a formidable sized army.