Many on the progressive left operate on the theory that the Second Amendment 'grants' the right to keep and bear arms.

In the view of many others the Bill Of Rights (including the Second Amendment) assumes many 'a priori' natural rights (including the right to self-preservation) and therefore places constraints and prohibitions on government from interfering with those rights. I agree with this view.

This article traces the concept of self-preservation from the ancient Greeks onward, and contrasts opposing views on how it's handled in societies. Here is one important paragraph:

"In the 2,500 years since Plato and Aristotle first debated the nature of rights and man, there has been a clear dichotomy on the question of self-preservation.  Those who deem the state paramount and, thus, the source of the rights of man believe that any right of self-preservation can and must be limited as the state sees fit.  On the other hand, those who deem certain rights, including self-preservation, natural or God-given believe that the state not only cannot limit or deny this right, but must guarantee it.  There is no middle ground."

That's right. There is no middle ground. Proceed accordingly.

Yes, I saw that article (mostly thanks to your pointing me to that website).  People often forget that the Bill of Rights doesn't say what we're allowed to do, but what government is not allowed to do to us.