In a Christian Science Monitor article about America's "gun culture", reference is made to an essay that suggests that with the 2A, the notion of civic engagement implies that people ought to "participate in . . . law enforcement and defense of liberty":

The pro-gun movement has been expanding ever since, aided in part by favorable legal rulings and writings. In 1989, Mr. Levinson, the Texas law professor, wrote a notable essay in the Yale Law Review in which he suggested that citizen participation in government might extend to the Second Amendment.

Levinson looked specifically at whether "ordinary citizens [should] participate in the process of law enforcement and defense of liberty rather than rely on professionalized peacekeepers, whether we call them standing armies or police." Gun rights activists consider it a hinge moment in the gun debate, since it marked one of the first such dissections of the Second Amendment by a liberal legal scholar.

That's quite a notion, especially for a liberal scholar (as the article notes).  Sounds right to me—there haven't always been so many professional LEOs running around.  The essay, entitled The Embarrassing Second Amendment, is available online.