Some of the citizens overthrowing the government would be in the military, though, no? Since the military is made up of normal people that live among civilians. It's more likely the military would split into factions, with the revolutionaries gaining control of a sizable chunk of tanks, airplanes, etc. Or maybe the military would stage a coup.
However, a truly tyrannical government would have long purged guns from its civilian population as evidenced by every tyranny ever.
It'd be a mess.
Very unlikely. Biggest part of military training is brainwashing you to always following orders. People that resist brainswashing are quickly discharges as "unfit for duty".
The US military has both Republicans and Democrats in it (though probably a larger percentage of the former). If there is any sort of internal conflict, it would likely split along those party lines. Also, the US military does not advocate blindly following orders. The person receiving the orders should only obey lawful orders and should resist and disobey unlawful orders ("I order you to go shoot all those unarmed civilians over there!")
Could you point out the brainwashing sessions in Navy boot camp perhaps?
I do remember a constitutional law class we all had to take where we were specifically taught that we had an absolute duty to refuse unlawful orders, maybe you were confused and for things backwards?
Look at the Chinese response to the protests in Hong Kong. Are they flying bombers over the city and setting up artillery? No, they're using forces of infantry, police, and paramilitary thugs who are far outnumbered by the protesters. If they actually leveled one of their own cities the entire government would collapse on itself.
However, given a sufficient percentage of the population which is interested in doing this, it would be very difficult even for the US military to completely conquer.