Do we really think that armed citizens would have a chance against the current US military? Weaponry has come pretty far since the days the 2nd amendment was created.
Also we really don't know what would happen if the US military was asked to start fighting its own citizens. There is no guarantee those orders would be followed... people don't like killing their neighbors even if they're ordered to do it.
To your latter point, there are constant human rights abuses/crackdowns on dissent in China, but does anyone intervene? Now consider that the US has military might many times China's. Who's going to intervene?
I, too, would hope for defection if the US military was told to start attacking US citizens. It's important to note, though, that the military goes to great lengths to break recruits down and rebuild them into "warriors" aka unquestioning killing machines.
See Vietnam
Would it surprise you to learn that the "US military" aren't a bunch of mindless drones, but are in fact family, friends, neighbors, co-workers and fellow citizens? And that they wouldn't necessarily feel great about slaughtering their fellow countrymen?
Trying to come with 2nd amendment is bullshit, you have to get the sympathy by the military. This could be by inflicting damage, but more likely by having high enough casualties that the military thinks it is too much.
It would be a bloodbath.