NRA has sold their members out before. For good reasons, in their mind. But still. Trump's not trustworthy, but there's no point telling his true believers that. He'd throw gun owners under the bus in a heart beat.
No-fly is barely legal, and flying isn't a Constitutional right. Using the no-fly list to bar people from Constitutional rights is possible, any sort of stupidity out of Congress is possible. But if enforced and upheld by the courts, it's essentially the death knell of the Constitution. Or the start of it at any rate. It means, not just for the Second Amendment, that any Constitutional right can be arbitrarily denied by 'secret' government lists with no accountability, oversight or due process.