I actually agree that the 2nd amendment exists for this purpose (protect against a tyrannical government) and think that is a good thing. An armed private citizen stands zero chance of effecting any defense against our military. None. Zero. It'd be a joke. Futile. Even the most practiced and prepared "militias" would fairly quickly be put down if the full might and focus of the US military where to be trained on them. The most a militia could hope for in such a scenario would be a guerilla warfare style protracted conflict combined with broadcasting a message that would win over the hearts and minds of the general public, that would then cause the US military to stop due to political, not military reasons. But such an outcome is unlikely in the scenario you're presenting, because the military would be controlled by a tyrant, not the public.
One thing to note is that under current law you cannot stockpile weapons. And you cannot create bio weapons or cyber weapons. You have no access to massive bombs. The 2nd amendment, in my opinion, has become an excuse for individual to own weapons that are dangerous to private citizens and the safety of the public but of no consequence in a fight against a tyrannical government.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Note the "a well regulated Militia" and "a free State"
Militia is capitalized. As is State. In politics (and generally internationally "State" has a very different meaning than is commonly used in the USA (in modern times, the founding fathers used it more in line with the way international law understands it to mean). To keep it short, the National Guard, again in my opinion, very effectively fulfills the purposed intended of this amendment. It is a well regulated militia that the State can direct to mount an effective defense against a tyrannical Federal government.