The Second Amendment to the U.S. Constitution reads: “A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.” 
On its face, this might appear to protect the owning of weapons for militia purposes. But the Supreme Court has authoritatively opined that the prefatory language about the militia, while it expresses the purpose of the constitutional protection, does not limit the scope of that protection.  The individual possession of firearms for personal protection is protected by the Constitution.
Thus said the Court in District of Columbia v. Heller, a 2008 case, which summarily dispensed with decades of scholarly opinion about the Second Amendment to the effect that the right to keep and bear arms is essentially the right of states to maintain a militia. That, of course, was never an entirely honest approach, since the Second Amendment specifically speaks of “the right of the people to keep and bear Arms…,” not the power of the states to organize militias.
But the Second Amendment has been a source of discomfort for some time, speaking to a milieu that has long ceased to be operative, and portending absurd outcomes in an age of advanced weaponry. The Amendment was born of the jealousy against standing armies that characterized the early Republic, at a time when the difference between military and civilian arms was not so marked. Nowadays few doubt the necessity of a standing military, and fewer still imagine that an association of armed civilians could put up meaningful resistance to the United States armed forces.
What never changed, however, was the natural law right of a person to defend himself against deadly assaults, and to be suitably armed for that purpose. It wasn’t the focus of the Second Amendment, doubtlessly because no one anticipated that the federal government would be empowered to disarm the entire population, but it was a critical right recognized by the English Common Law that we inherited. 
What the Heller case did was dislodge the right to keep and bear arms from its militia moorings. And even if one can argue that it did not do so legitimately, it certainly did so effectively, for such is now the state of the law. But this is not to be lamented, because prior to Heller the law was, quite arguably, that the Second Amendment protected against banning possession of the standard military rifle , which nowadays is the fully automatic M-16. But after Heller there is no longer any doubt that such weapons can be prohibited for civilian use.
That means that civilian possession and ownership of the kind of weapons used by the Las Vegas shooter can constitutionally be prohibited. What’s more, now that the Second Amendment protects weapons ownership for self-defense solely, a reasonable restriction on the number of firearms a person owns could be enacted.  And it really needs to be done at long last. There simply is no sane argument that semi-automatic rifles with high-capacity magazines should be available to the general public. A restriction on the number of firearms one person may own is also necessary, so that a disturbed individual like the Las Vegas shooter cannot substitute a number of firearms for a high-capacity magazine.
There is, of course, the old argument that criminals don’t care about the law, and will own prohibited weapons regardless of what the law is. But that is only true to a point. We can expect the same sort of individuals who already traffic in contraband to gain access to prohibited weapons. But, strange as it may sound, those are not the individuals who would be directly targeted by such a law. The ones who would be targeted would be the sort who feel compelled to shoot concertgoers, or children in schoolyards, and there is no reason to suspect that individuals operating with that sort of mental disturbance would have underworld access.
The incident in Las Vegas is really the last straw, if it didn’t arrive before now. There is a serious need for strong legislative action. Now.