You'd be surprised how many military members would stay and kill their own civilians. I'd say right around 50%. Which is enough. A single marine division would be enough to defeat the entire U.S. A hillbilly with a 12 gauge isn't going to do shit against the most powerful military on the planet.
Also, the U.N. would do a whole lot of talking and a whole not of walking. Russia kills their own citizens every day. The U.N. scolds them and doesn't give a shit. The reason the U.N. intervenes in countries in the Arab spring is because they are little ass powerless countries. No one is going to do anything should America turn on it's people. No one is going to risk war with the U.S. besides China or Korea and they sure as hell aren't going to support a revolution. The U.N. would slap us on the nose and scold us and go about their day. Happened when we went into Afghanistan and would happen again. The U.S. is the action hand of the U.N. without us they won't do anything but talk.
With weapons like an AC-130, A-10, Reapers, F-35 and Ospreys an AR isn't going to do shit. A single drone could destroy this entire nation if all we have are ARs. They can stay in flight for days and fly ten miles in the air. From that distance they can recognize a face in a crowd with very good certainty and then shoot them in the head with a single round. No one would hvae any clue it happened or even how it happened. And this only requires a dozen or less people to operate and maintain these things. And don't forget these things can carry a payload around 1,000 Ilbs too.
Defense against the government is not why you need firearms. Now let's get back to arguing about guns.