1. The U.S. military is ridiculously well financed and supplied. Even if every American strong enough to lift them had a battle rifle and 200 rounds of ammunition, the federal government has mobile armor, fighters and bombers, and guided explosive munitions. The average American simply does not have the means to fight toe to toe with any professional army. As guerilla fighters, it is usually more useful to build explosives and make booby traps than it is to fire rifles. And if you're close enough to hit something with a rifle, you should probably already be running away.
2. We have strong gun rights now. They have not perceptibly slowed the government's slow ratcheting towards greater power and Constitutional overreach. Whether the government is already tyrannical is a matter of opinion, but be assured that some people think that it is.
3. Guns are not the only tools required to resist government. They are neither necessary nor sufficient. Defending gun rights in preference to any of the others, such as privacy, speech, assembly, redress, and due process, is to abandon our best means of resisting government power in favor of the one least likely to yield a predictably favorable outcome. All human rights work together to produce liberty, and none are more important than the others. To prioritize them is to see them picked off one by one, back to front, like Alvin York would.
I think 3 is an excellent argument but is not really a hole in the idea that gun rights are necessary to prevent tyranny.
Lots of people don't see the NSA's activities as "illegal spying on its own people," but rather as strong anti-terror efforts that don't give up when terrorists enter the US.
The idea that we need strong gun rights as a way to stop the government from becoming tyrannical has at least two major obvious holes in it, but a lot of people's views are more subtle and nuanced than that. As usual.