1) Those are the guns they are talking about banning. 2) Those are the guns about the banning of which they are talking. 3) Those are the guns whose banning they are talking about.
Gratefully, Navi.
Top answer
navitasan Which are correct 1 and 3.
— Mister Micawber
navitasan Which are correct 1 and 3.
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3a is not something you should ever use. In your first post, 3 is grammatically correct (your question was about "correct" not "natural") but something no native would ever spontaneously say.