Its clear from the text of the 2nd, and especially from the SCotUS interpretation of that text, that the 2nd amendment protects an individual right to keep and bear arms, with the exercise of same being independent of any relationship to any militia.
The question then becomes what sort of regulation can be placed on the right to arms without conflicting with the Constitutional imperative that the right to keep and bear arms shall not be infringed? That is, what regulations can the government lay on the exercise of the right to keep and bear arms without infringing upon that right?
It seems to me that we have a rather broad set of examples to look to for guidance -- the strict scrutiny jurispridence surrounding the various rights protected by the 1st amendment, specifically the right to free speech, the freedom of the press, and the freedom to assemble.
Without going into detail, the general direction of this jurispridence is that these rights do not include actions/expressions that cause direct harm to others, or place others in an immediate position of clear and present danger -- you may freely express your opinion of someone, so long as you do not slander them or commit libel; you can advocate individual or collective action so long as said action does not include things like inciting a riot, and you can make public proclimations/exclamations, so long as you do not directly endanger others by doing something like yelling 'Fire!" in a theater.
These conditions placed on your first amendment rights create excellent analogues for constitutionally acceptable conditions for the exercise of the right to keep and bear arms -- you may freely exercise your right to keep and bear arms so long as you do not cause harm to others (outside the obvious exercise of the right during the exercise of right to self-defense) or place them in an immediate position of clear and present danger; any restrictions above and beyond these create infringements on the right to arms and thus violate the Constitution.