Our government has the firepower to become tyrannical if it truly desired to do so. No tyrannical government with the kind of firepower the US has could be defeated by a ragtag group of plucky rebels simply because they have guns.
We have the second amendment and all of the amendments, however, because we have a system of government that prevents itself from becoming tyrannical. It has limited it's own power and allowed power to be retained by the people.
The second amendment doesn't prevent tyranny, it exists because of a lack of tyranny. It must be preserved because it's removal would be a sign of tyranny, not because it's removal would lead to tyranny.