I'll invariably make a few statements in this diatribe that I hope don't color my argument negatively, but I feel might help in clarifying my position.
1) I think that all arms that the government possesses ought to be allowable by the citizenry. That is my interpretation of the intent of the second amendment, though I do not necessarily discount other people's beliefs as wrong. I understand that the founders aren't still around to ask (regrettably), but given what I know of the times, and what I know of our war for independence, I see a couple of logical conclusions from that, and I have settled on the opinion I have because I believe it to be the most logical extrapolation.
I also acknowledge arguments that suggest that the second amendment was primarily meant to deter the ability of foreign invasion, as it has been proven to be effective for, and fits with the actions of the times (if not necessarily the 'off the record' statements of many of our founders), and I agree that it is not as effectively spelled out as I would like it to be. That said, none of the other rights enumerated by the Constitution are either, and people don't generally balk at them. Nowhere in the Bill of Rights is there any mention of Twitter, Facebook, email, megaphones, etc. I personally believe in the principle of liberty (and am in fact a Libertarian), and believe that people ought to be free, and that our country, as intended, does a better job of this than most others. That is not meant to believe that I think we are the best country, or that our people are in any way better than other people, but I think that, if our government works as it is supposed to, we have the greatest potential for freedom of any of the countries I am aware of.
As for where I set the lines, I believe that until its appointment process is somehow violated, the Supreme Court are the Constitutional authority of our nation. So, by that logic, I define my categorizations by what the Supreme Court has tested, and what they have remarked on. Regrettably, there hasn't ever actually been a test of the firearms act in the supreme court, so I am free to have a vaguely defined opinion of it because there hasn't been any opinion offered. Mayor Michael Bloomberg recently banned big gulps in the state of New York -- just because it's a law does not mean that I am obligated to agree with it. More importantly though, the courts recently struck down the ban as being arbitrary and capricious, which (thankfully) sides with my own personal opinion of it, so I believe that justice was done, however delayed.
I would happily keep going, but I think I've given enough to digest at least, and I hope I've explained my position well without completely convincing you that I'm one of those gun obsessed nuts you see on the news (I'm not).