My impression of the 2nd Amendment is that it was really meant to keep the government from ever having the force capability to even TRY to be tyrannical. Madison, for example, talked about a ratio of about 1:25 between men in the federal standing army vs. men in a potential civilian militia. Of course, this was back when the one-to-one capabilities were relatively comparable.
Now of course the one-to-one is nowhere close. In fact it is essentially irrelevant. The only way a civilian militia would slow down a modern army unit is if the army ran out of bullets and gas.
And so that force capability ratio has been blown past a long time ago. We now have the most powerful and capable standing army the world has ever known, and though we may have a lot of guns, the number of people that could actually be formulated into some sort of useful militia is probably pretty small.
And so the 2nd has completely failed its intent. It wasn't meant as a thing to win a war against a standing army, and it would now be very ineffective at doing so. If some group of yahoos with guns seriously challenged the government I think the government/police would in fact defend themselves. It would probably be relatively easy to frame that encounter Waco-like, as in we had to defend ourselves against a bunch of crazies with guns.
If we really had to overthrow the government, the most effective way would be a mass peaceful uprising. I can't see any police force or military force turning their guns on peaceful citizens en masse. Egypt, recently, serves as a good example of this in action. No 2nd Amendment required.
It is bewildering to me that many of the people who seem most concerned about the 2nd Amendment as a defense against government tyranny are often the same people who argue for giving the government extraordinary power (e.g an ultra-powerful standing army and policies such as the Patriot Act). It seems paradoxical to me.