With all the firearm owners in the us it would be dam near impossible take everybody's guns but they can try. it would be unfortunate if it come to that as many people I have talked to would die before they give them up this really should not even be a topic but the powers to be make it this way oh and by the way my spelling is probably off who cares.
They don't have to take them. They won't come knock on your door. They'll just start by making certain firearms illegal. Like they do in California. They'll offer an amnesty program for you to turn them in and dispose of them. My guess is they'll pay you for them so that people can't complain about an illegal "taking". Just like they do with land condemnations. Severe penalties will apply if you're caught with them afterwards.