I'm always up for common-sense. Of course, things would be much easier if we could all agree on what that is.
First, I won't get into a debate about what the 2nd amendment really means. That is a question legal interpretation and history, not necessarily common sense.
Regarding your assertion that banning ALL guns would cause crime to fall at least 90%, I did a quick google search and received interesting results. It is clear that people on both sides try to cherry pick and, I don't doubt often, manipulate data to support their preferred conclusion. Again, it is not at all clear that there is a consensus on "common-sense".
Here has been my common-sense. Reducing the weapons around the world would certainly make it a better place. I'd start with the militaries and their weapons of mass destruction. Ideally, we would only disarm people who intend to use weapons aggressively and leave them only in the hands of people who will use them defensively (probably with allowances for hunting and sport). Of course, determining that is the trick. Examples abound in history when tyrants tried to disarm their victims with the intention of more easily subjugating them. We should be cautious of the people who try to disarm us.
Furthermore, if you were the gun-control czar, would you be willing to stand accountable for your determination. If after disarming non-military personal, your military comes under the control of a tyrant and is used to commit war crimes, will you be held responsible? If you disarm a peaceful, law-abiding person who is later killed by a criminal with no regard for your policies, will you answer to the family of the person who might have defended himself?
You know, that is not the way that political control works. The people who implement gun-control policies, even assuming that they are benevolent, will not in the least be held accountable for them. Worse yet, political control attracts people who use it for personal gain. How long before a disarmed society comes under the control of someone truly dangerous?
If you want common-sense, and if we’re able, let’s disarm people who have demonstrated that they will misuse them. Your assertion that crime would fall 90% (within 24 hours, no less – as if criminals only obtain guns just before committing crime) is ludicrous at best. It is an indicator to me of someone who does not think through very well. Your assertions are far from common-sense.