AR-15s aren’t in the Bill of Rights, either. No weapon is specified, just that people have the right to bear arms; the arms of the day were black powder, muzzle loaded Kentucky rifles. The only relevant difference between a Kentucky rifle and an AR-15 or an AR-15 and a nuclear weapon is in the magnitude of death dealing ability. They are all arms. Wherever you draw the line as to what is and is not acceptable is arbitrary. You seem to be OK with not being able to buy a nuclear weapon, but you also seem to be OK with being able to buy an AR-15. So, your personal, arbitrary line is somewhere between AR-15 and nuclear weapon.
As for the Bill of Rights being old, of course that makes it outdated. Just like all other legislation that is centuries old. It’s just a document written by people; it is neither sacred nor perfect. You either update it, or you find yourself living in a society with archaic ideas of right and wrong. The idea is to do that sooner rather than later, because the disparity between how things were in the era of the Kentucky rifle and how things are in modern times only grows as time goes on. The AR-15 will seem like a squirt gun compared to the cutting edge personal firearms of tomorrow.
The question is whether you can acknowledge that weapons today are able to kill more people in less time than weapons in 1791, and whether you can acknowledge that, in all likelihood, weapons tomorrow will be able to kill more people in less time than weapons of today. In that context, it is illogical to defend the Second Amendment, because the inevitable march of technological progression will have people armed with weapons orders of magnitude more deadly than an AR-15, where shooting sprees claim the lives of thousands of people, not “just” 5, or 10, or 50.
Whether I am personally familiar with firearms is irrelevant to the morality of these questions and, as such, has no bearing on what the law should say. Maybe I am. Maybe I’m not, but my position is that the Second Amendment should not be used as a crutch to enable mass murder. You can argue all day that murder is illegal, but that doesn’t stop it from happening, and the more powerful weapons you legally and easily put into the hands of murderers, the larger the scale of the mass murder. I doubt anyone would have much luck shooting up an entire school or nightclub with a Kentucky rifle, so I don’t see the Second Amendment as immoral in the context of 1791 society, especially when you consider a novice couldn’t even load and fire weapons of that era. Things have changed, though, and clinging to legislation penned a quarter of a millennium ago is clearly suboptimal if you care about human lives.