Funny. I googled Government taking guns from law abiding citizens and couldn't find any cases. Then I realized if there was a even a tiny case of it, You guys would be blasting this forum with links to it.
No difference between banning or confiscating without grandfathering existing owners. Law abiding citizens who owned any type of machine guns, such as the Thompson, were considered felons after 1936. There's a case for you. Move along now.