own weapons with such massive killing power to protect against tyranny by the US gov't is an idiot. The US military has such overwhelming force and technology, that there's no amount of firepower that ordinary citizens could possess to protect against a tyrannical gov't if things went south and the military sided with the gov't.

It seems there are a handful of reasons why people own AR-15s:

1) It makes you feel like a bad ass and makes for awesome Facebook pics posing with your kick-ass gun. Or in the words of Jim Jeffries, this is known as the "fuck off, I like guns" justification (It sounds better in an Australian accent)
2) You're a paranoid asshole who thinks marauding bands of hoodlums are going to kick down your door in a home invasion at any moment and a handgun, shotgun, or bolt action rifle is not enough to take down the sheer number of potential assailants.
3) You're a delusional idiot and think you could last more than 5 seconds against a highly trained US military in the unlikely event you needed to fight against the tyranny of the US gov't. The venn diagram of #2 & #3 contains significant overlap
4) Your farm is overrun by feral pigs.

Only #4 seems like a valid reason, and I'd be fine with HIGHLY regulated exceptions for people who can demonstrate this legitimate need.