In case you were not aware the watch list and no fly list are already a mess. Patriot and DHS are already being used for all sorts of things that have nothing to do with terrorism. Your profile says you are an attorney look up ‘parallel construction’ for a sample of how a regulatory agency will use powers granted to it to expand them to other areas outside their purview. So on principal it would be a bad idea.
But in effectiveness? The Orlando shooter was cleared on his FBI investigations and was not on the no fly list so even that would not have prevented this tragedy. Again he faced stronger scrutiny than any proposed legislation would have implemented on the millions of legal gun owners in this country. And he passed with flying colors. Here I will cite Bloomberg so no pro-gun bias can be used as a complaint: