I think the US government would be pretty much powerless against an armed insurrection by 50 million people.
A governments power is not blowing people up, it is extorting taxes out of them. Syria will fall, not because the rebels are well armed but because the conflict has bankrupted the government.
A sea of weapons means the government forces are not safe anywhere, hence the costs of security go through the roof while tax revenues go through the floor.
This is a rather simplistic view of a country where government can and does influence the average citizen in ways we cannot comprehend even if we wish to.
It is not at all hard in our PR capable society to turn one segment of the population on another. We see that in many movies where ordinary folk are asked to turn on their tvs and watch "deserving" criminals attempt to escape. "look our your doors, look out your windows and call us with what you see - there, or over there, is the hideous man who killed 12 police officers in cold blood, you saw it with your own eyes". Never mind the truth, that the running man was killing in self defense, that he is simply trying to win money for his ailing child and that the game is rigged from the start - rigged in order to keep the population in line by lauding them as law abiding citizens and all others as the enemy.
Not hard at all to show shorts of bearded beer drinking militia staging mock defensive maneuvers while telling honest citizens that they are flaunting the law.
You seem to believe that these millions are well trained, well organized, communicate well with some unnamed central command and have all the time in the world to gain whatever objective they wish.
And what might that ultimate objective be? What is their goal? it is easy to say "to overthrow the current government" but what does that actually mean, what will signal their success and what will they put in place of the newly overthrown government?
It is a wonderful, heartening, and noble idea but it has no real meaning any more, the time when we as citizens might have overthrown our government is long past. The power remains with a people only if they know full well that their guns will not alter things, when they are fully aware that they must use other means to accomplish their ends.