But the US federal gov't already forcefully takes guns away from people when they are breaking the law.[1]

If that law is changed to include guns that are now legal, why would you think the ATF won't seize those guns too?

Not long ago the ATF determined that certain airsoft guns were "machine guns". They pretty quickly asked the seller for a list of customers, followed up with each and seized them.

[1]https://www.usatoday.com/story/news/politics/2017/12/04/excl...