Well, consider the implications of bombing someone on American soil. What about collateral damage? What about anyone else who wasn't on the crazy guy's side, but seeing him summarily executed, untried...for what? The argument from this angle is as unreasonable as EXPECTING the government to start slaughtering citizens on American soil in the first place.
If we were to ban guns, there will undoubtedly be people holing up in their houses, exercising what was, until that point, their freedom, as explicitly written in the bill of rights, for this express purpose. And those people would be spurred by a brazen attack, to say nothing of the fact that it is in direct disregard of the sixth amendment(and Geneva conventions, making it a war crime) This would be a big can of worms that we can't simply disregard so we can pick up a nice talking point!
Suffice it to say, any government who bombs its own civilians on American soil without a /very/ compelling reason is as tyrannical as the crazy guy thinks. I'm pretty sure if that was an option, they'd be using it.
This is how the logic works, then -- if they can't bomb them, they'd have to use small arms, or the national guard with its larger small arms, and as such, their own small arms would work against them. This is especially true in civilian centers, where the risk of collateral damage is high, in both resources and innocent lives.
Then, how would it be seen on the international stage, for the US to execute civilians en masse without a trial? They'd be talking about US in the same way we talk about countries who do, literally, the same thing in Africa and the Middle East. We would instantly lose any credibility there, which may have disastrous effects on our economy, be it through sanctions, withdrawal of treaties, withdrawal of trade agreements. Even IF the government were tyrannical, it'd be nearly suicidal to do that. I'm by no means an anti-statist or what-have-you, but it'd be a bad decision to bomb people on our own soil, even outside of the moral implications.
Then we'd need to start a witch hunt for anyone with even somewhat similar views, because if you thought Waco was bad, imagine more people, just as crazy, acting against what are, by definition, declarations of war, with access to the internet, in rural and subrural areas that are most likely to have large amounts of components to build explosives.
I have to say, it's a fun thought experiment, but it wouldn't be fun if it happens. Best to do what Australia did, and pressure the holdouts to abide by the rules until they do, instead of just killing them.