There are about 1.5 million active and another 1.5 mil reservists in the US Armed Forces. Most of those would have a serious problem with turning military weaponry on fellow Americans.
The few that did, even with a massive technological and armament advantage, they would be in for a hel of a battle.The 70-80 million people in the US that own a gun make a fairly formidable foe. That isn't even considering the creative types making other weapons.
Tyranny has been held at bay and our constitution has stood for a couple of hundred years. You can't say that it doesn't work.